Why is Betelgeuse named after a star?

Beetlejuice was named after the Betelgeuse Star, a star in the Orion constellation. The reasoning behind this is that Betelgeuse is supposed to be the doorway to the infinite blackness of outer space, a metaphor for Beetlejuice being the doorman of the underworld.

The rest of the answer is more about the switch between the name of the antagonist and the name of the movie itself and its pronunciation and includes none to little guesswork about the reason behind the reason for the antagonist's name originally being Betelgeuse.

From Indiewire

The title for “Beetlejuice” is a play on the character Betelgeuse’s name, which in turn based on the Betelgeuse star, which is housed in the infinite blackness of space but in the same constellation as the more famous star Orion. (Screenwriter McDowell was surprised anyone picked up on the reference.) Warner Bros, however, didn’t think there was anything clever or funny or interesting about the movie’s title, and begged Tim Burton to allow them to change it. He refused.

The alternate title the studio had come up with was “House Ghosts,” which, at the very least, beats out “Anonymous Haunted House Story 39480,” which more or less conveys the same general mood and aura. Burton, the story goes, suggested “Scared Sheetless” as a joke (a reference to the scene where the Maitlands attempt to scare the Deetzes out of the house by wearing bed sheets – something they consider spooky but comes off as utterly laughable). Much to Burton’s horror, the studio actually liked his idea, and tried to rename the movie. Burton finally put his foot down and said that the movie would be called “Beetlejuice.” Once and for all.

I've found also these two sources on the subject which include more straightforward answers to the question and have correct astronomical references :)

From Movieplot:

This guy's name may be pronounced as Beetlejuice, but it is actually written as "Betelgeuse" (named after the 9th brightest star in the night sky). The reason that director Tim Burton decided to title the film 'Beetlejuice' opposed to 'Betelgeuse' is actually very straight forward.

A: Beetlejuice sounds funnier (for the younger audience)

B: Beetlejuice is a lot easier to remember, pronounce and spell compared to Betelgeuse.

The film studio actually disliked the title, and considered renaming the picture "House Ghosts." As a bit of a Mickey take, director Tim Burton put forward the idea "Scared Sheetless," but was horrified when he discovered that the studio was actually considering it!

So, as you can imagine... the original intent of Tim Burton was to name the film "Betelgeuse," but they later decided on the name change for the movie, but not for the title character, hence why we see "Betelgeuse" in the TV advert and gravestone. Confusing for those who don't know, but actually pretty straight forward for those of us who do!

From Beetlejuice Wikia:

Betelgeuse's name is often spelled phonetically as "Beetlejuice", as in the title. However, during the film his name spelled Betelgeuse (like the star Betelgeuse) during his ad on TV, and everywhere in the grave scene.

There has been great debate over which spelling of his name is correct. Some say that because his name is the same as the introduction, this is proof they meant it as Beetlejuice, while others claim that was an error as the credits of the film spell his name "Betelgeuse". Nowhere during the film is his name spelled Beetlejuice, unless watched with subtitles on, in which it is said from Lydia's interpretation from playing charades with him. The Maitlands used the astrological spelling... after several mispronunciations.

Some also argue that he may not have been permitted to write his name the way it should, much like he can't say his own name. However, it is just as likely that he cannot misspell his own name any more than a mispronunciation of it can summon him, a condition that compounds his curse since his name is not spelled phonetically. It is also worth noting that there are hints of a greater meaning in several of the names in the movie, since Betelgeuse means "the hand of Orion," and Betelgeuse's former master was named Juno, a character clearly named after the Queen of the Roman gods, protector and special councilor of the state.