Because he was an unnecessary addition to the story, completely out of character for what XF was about. The whole baby William story line changed a fantastic show into a very sappy and boring soap opera. I have nothing against the kid in general, I just hate what his story did with my favourite show :(
I don't hate Baby William. And just to let people know if you were true fans and the entire series of The X Files, he was an important key to the series especially the whole Mulder and Scully storyline. The Baby that really gets on my nerves and that i hate is Baby Christina on Bones that's who you should hate she was unnessary to the Bones storyline. At least baby William had a purpose baby Christina didn't she was just an easy and lazy way to put Emily Deschanel's real preganancy in the show which was unnessary anyways because they could of at least hid her pregnancy like most shows do.