why was Arwen a main character in the movies? In the books she was only at Rivendell or occasionaly talked about by aragorn.

I'm not sure, but I think when they wrote the first book they wanted to add her in more (more females, more love interest for aragorn, etc).
I know why they kept her for Two Towers and Return of the King though. Liv Taylor (the actress who plays Arwen) had gone through a ton of training to be Arwen because they had written her in as a fighter. However when the fans rebelled saying exactly your point (she is in like two scenes in the series and vaguely talked about by aragorn) they decided that they wouldn't just completely take her out of the film so they took notes and stories from the appendixes of lord of the rings.

They made Arwen a main character because they wanted her character to be more popular than in the books

Interesting question. Well,maybe because Aragorn's part in the movie was big and somehow his destiny was bounded to her. She gave him a strenght whenever he had felt weakly...

Obviously, because they wanted to add a love story to the films, in order to attract romance lovers. But Tolkien himself wrote the Lord of the Rings as a "male" book: that came from his past in Oxford University, when all the groups and clubs were composed of men only. They chatted together on various intellectual subjects and spent time away from their wives or girlfriends. Tolkien always enjoyed these gatherings really much and he wanted to reproduce this in the books: that's why the Fellowship is composed by men only, and it's also why all the great actions are accomplished by males in the books.

i have no idea because your right in the book she was only mentioned a couple times and always in rivendel

Ahahahahahahahaha, as someone who saw the movies about a dozen times before reading the books, I found this absolutely hilarious ^_^ She pretty much didn't exist in the books ^_^ I guess they thought it would be nice for Aragorn to have a proper love interest, because there wasn't really any romance in the story.

Because they wanted to shoehorn female character and love story in the movie as much as possible to attract girls who likes romances. I would prefer if Arwen didn't steal Glorfindel's part and if Faramir/Eowyn romance was in the movie instead of so many Aragorn/Arwen and Eowyn/Aragorn scenes.