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 do suggests that "boastful one" would fit best.
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.
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.