Because in Greek mythology the Morning Star was the son of the goddess of the Dawn (and serial rapist) Eos (Aurora to the Romans) the Greek Septuagint translators of Isaiah 14:12 chose to render the personage 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 Helel who was the son of Shachar 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.
Shachar 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 male god associated with the Dawn, his brother Shalim being Dusk. Shachar and Shalim were among the sons of El Elown.
There are NO texts outside Isaiah 14 that identify Shachar as having a son named Helel.
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 Helel nor is he ever refereed to as a son of Shachar. 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 Ashtroth, or her son. Astarte was a sister/wife of Hadad and daughter of El. Hadad would probably be the father of any children of hers.
So the morning star and the Dawn in Caananite mythology were siblings or maybe uncle/nephew but not father and son.
There is a Canaanite myth about Attar rebelling against Baal Hadad but NOT El, Hadad himself was the rebel against El.
Isaiah 14:12 is the only verse to use the word Helel. But Helel is 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. The context of mentioning dawn implies shine works best.
A more accurate Greek rendering could be Phoibos/Phoebus which means bright or shining one which was an epithet of Apollo given to him after he became affiliated with the Sun (he originally was not). Or Phainon, a Greek name for the planet Saturn which Cicero says in On The Nature of the Gods meant "Shining".
However that Boasting is very much what this personage goes on to describe him doing suggests that "boastful one" would fit best. Interestingly Bromius, a name for Dnonysus, means "noisy", "roaring", or "boisterous", from βρέμειν, to roar.
Helel may also be Yalal (Strong number 3213) with a definite article. Making it a title not a name. It means Howl or Howling. So as a title would mean Howling One.
The New Testament refers to Jesus as the Morning Star (Revelation 22:16) and the Day Star (2 Peter 1:19). The Day Star reference used a poetic name for Venus "Phosphorus" which refers to the same thing Heosphorus does, and has the same meaning Lucifer has in Latin. Phos=Lux=Light and Phorus=Ferus=Bearer/Bringer.
I've seen it described as though the Septuagint uses "Heosphorus" for the entire phrase "Helel ben Shahar". So the Heosphorus being a son of Eos in Greek mythology is probably he origin of this mistake, since other mythology don't make the Dawn a parent of any stars. Interestingly the etymology of Heosphorus perhaps makes a better translation of the "Ben Shahar" part, meaning "Dawn-Bearer" or Born of Dawn.
I've been thinking however, what is Dawn isn't even an accurate translation of Shahar here? It definitely means Dawn in many places, but a Hebrew word spelled the same also means "Back" or "Dark". When Shulamiht is described as being Black like the tents of Kedar (Song of Solomon 1:5), Shachor is the word translated black. Perhaps this similarity makes a certain kind of sense, "the Night is darkest just before the Dawn". Maybe Satan is actually being called the "son of the Darkness".
So calling Satan the Morning Star fits his agenda quite well. The Latin Vulgate indeed uses Lucifer in both Isaiah 14 and these NT Morning Star references. Because of this there are some Latin Catholic hymns that call Jesus Lucifer which ignorant protestants have had paranoid reactions to.
And maybe even identifying Satan as an offspring of the Dawn is dangerous. Because one could easily argue if they wanted to 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, 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.
Gnostic and Jewish enemies of Christian could make use of such arguments.