Because in Greek mythology the Morning Star was the son of the Dawn goddess Eos (Aurora to the Romans) the Greek Septuagint translators of Isaiah 14:12 chose to render the personage seemingly identified as a "son of the Dawn" as Heosphorus, the Morning Star. Which became Lucifer in Latin versions like the Vulgate.
Repeatedly people will tell you that scholars believe Isaiah 14:12 and up references a Canaanite myth about the god of the Morning Star named Heylel who was the son of Shahar god of the Dawn rebelling against El Elown. (Elown is the Hebrew title translated "Most High" or "The Highest".)
What they won't tell you is they have no actual text or inscription describing that myth with those names. It's all conjectured from their assumption that Isaiah 14 must be drawing on some kind of Canaanite myth. The rebel against El in the Ugaric Baal cycle was Baal.
Shahar is the Hebrew word for Dawn (morning in the KJV but that's unfortunate because it's not the standard word for morning, Dawn is more accurate) used in Isaiah 14:12. That word is also the name of a pagan Caananite (assumed to be male) god associated with the Dawn, his brother Shalim being Dusk. Shahar and Shalim were among the sons of El Elown. But there are NO texts outside Isaiah 14 that identify Shachar as having a son named Heylel.
Attar (also rendered Ashtar, Ishtar, Astar, and Athtar) was a god affiliated with Venus the Morning star. But he is not associated with the name Heylel nor is he ever refereed to as a son of Shahar. He was a male counterpart to Ashteroth/Astarte, who's name is similar and was also affiliated with Venus. We don't know for certain his position in the mythological genealogy but I'd suspect he was like a brother maybe even twin of Ashteroth, or her son. Astarte was a sister/wife of Hadad and daughter of El. Hadad would probably be the father of any of her children.
So the morning star and the Dawn in Caananite mythology were siblings or maybe uncle/nephew but not father and son. There was a Mesopotamia Dawn Goddess named Aya but none of her children were Stars or Planets, that's a Greek thing.
The insistence of scholars that the Greek tradition of the Morning Star as a son of the Dawn must also be paralleled in the Near East is a product of Western Chauvinism, our tendency to filter all other pagan mythology through the basic framework of Greek Mythology. It's something I my myself have fallen into when studying the Shinto Pantheon to help me understand all the Anime I watch. But at the end of the day the fact is Joseph Campbell was wrong, different cultures do have different ways of thinking about these things, they aren't all the same Hero with a Thousand Faces.
Isaiah 14:12 is the only verse to use the word Heylel. But Heylel could be just the noun form of the verb Halal (Strongs number 1984), which has a variety of meanings, shine, boast, celebrate, glory, praise, rage, mad, all words the KJV has rendered it as. Heylel could also be Yalal (Strong number 3213) with a definite article, making it possibly a title not a name. It means Howl or Howling, so as a title would mean Howling One or Howler. Isaiah uses Yalal elsewhere in this chapter and in the prior one and many other places, however Halal is a more rare word in Isaiah.
English Translations of the Peshita version of Isaiah 14:12 don't even interpret Heylel as a noun but as a verb saying things like "Wail at Dawn" or "Howl in the morning" with the entire "son of" part completely absent. Given how ambiguous the grammar in Isaiah's poetic style can be, I'm starting to think the Peshita may be the correct reading and that this statement directly corelates to Revelation 12:15.
Actually I have discovered that the Septuagint reading doesn't contain the Son of designation either, nor does the Vulgate, both saying "Rise in the Morning" or "Morning Rising", however the use of Ben the Hebrew word for Son is in the DSS Isaiah Scroll so it was there in the BC era.
This Website documenting the DSS Isaiah Scroll
Classifies "Shining One" as an example of a spelling difference that doesn't effect the meaning, but I am skeptical given how disputed the word is. It seems the Great Isaiah Scroll spelling of Heylel adds another Yot. And that's a surprise because usually the DSS manuscripts use less Yots then the Masoretic because using Yots sometimes as vowels wasn't a thing yet.
The standard Masoretic spelling is Heh-Yot-Lamed-Lamed but in the Isaiah Scroll it's Heh-Yot-Lamed-Yot-Lamed. When I put the that DSS spelling into Google Translate it comes out as "The Praise" or "The Blasphemer", if that is correct it verifies a Halal based interpretation over Yalal. However the last half of the word looks like Layil the Hebrew word for Night.
Other Isaiah Scroll differences in Isaiah 14:12 include the lack of a definite article before Heaven, and Nation being singular rather then plural, which better fits the early part of the Chapter where it's about an Oppressor of Israel specifically.
There are only three verses where Halal as a verb is translated Shine or Shined in the KJV, in each case it described an action of a Light or Light source where poetically the Praise or Praised meaning of Halal could in my view fit just fine. Hillel is a Hebrew name derived from halal with the Yot between the Ls but not between the H and L. That name is always interpreted as meaning Praising. So no even if Halal is the basis of Heylel "Shining One" is not a valid translation.
I'm still unsure what the words used in Isaiah 14:12 exactly mean, but no valid Hebrew etymology supports HYLL or HYLYL meaning Light-Bearer or Morning Star.
The KJV New Testament refers to Jesus as the Morning Star (Revelation 22:16) and the Day Star (2 Peter 1:19). In the Greek the Day Star reference used a poetic name for Venus "Phosphorus" which cosmologically referred to the same star Heosphorus did, and has the same meaning Lucifer has in Latin. Phos=Lux=Light and Phorus=Ferus=Bearer/Bringer.
I wonder what those Hebrew Roots people who think the entire NT was actually first written in Hebrew think Peter called Jesus here? The Hebrew equivalent of Phos is Owr the same word for Light used in Genesis 1:3. Phorus/Pheros as a word for "bear" means in the sense of to carry something and isn't ever used of say childbirth or pregnancy. So it's best Hebrew equivalent is probably Nasa which first appears in Genesis 4:13, 7:17 and 13:6. But there is no known Hebrew compound word like that in antiquity, this concept is from Greek astronomy and so it's presence in the Epistle in my opinion itself refutes the possibility of it originating in a different language.
The title of Morning Star is most often given to Venus but other planets can be visible as Morning Stars in the right circumstances including Jupiter. I of course don't think the main purpose of either of these NT verses is to make a specific Star symbolic of Jesus more then any others, I'm not arguing some type of Christian Astrology. But I have decided the mystery of the Star of Bethlehem is in some way a Planet serving as either a Morning or Evening Star, but I don't have a final theory on it yet.
Psalm 110 is the most quoted chapter of the Hebrew Bible in the New Testament, always in ways that confirm it's subject is Jesus. Verse 3 refers to Him as coming from the "Womb of the Morning" with the word for "morning" being a form of Shahar. So even calling the villain in Isaiah 14 "Son of the Dawn" may be giving them a title that rightfully belongs to Jesus, or at least can in different ways equally apply to both.
It's possible to argue that the Woman of Revelation 12 is being described with Dawn Goddess imagery. Eos is frequently depicted in Greek art and poetry as wearing Saffron robes, Saffron is a shade of the color yellow that is commonly identified as being the Sun's shade of yellow. And since the Sun rises as the Moon is setting one could also say the Moon is under her feet.