Here's the answer to why: because Meyer did not write this book to be the next great American novel. Because it has become such a teen sensation, everyone loves to criticize her. Personally, I don't see why people become so pompous about what books they read; books, like movies or television, are meant to be a form of entertainment. Yes, I read Jane Eyre, and it is one of my favorite books. But I also read Twilight and enjoyed it immensely. Not because of the hype surrounding it-- I actually read it in 2005, before it was broadly known-- but because I loved the story of the impossible love between Bella and Edward. (Oh, and book Nazis: ever heard of Romeo and Juliet? By William Shakespeare, playwrite extraordinare, about two star-crossed lovers who cannot be together? Maybe Meyer's writing is not quite up to Shakespearean level, but the concept is the same. Yet if someone here was talking about Romeo and Juliet as their favorite book, I doubt you would bash them and call the Bard's work trash.)
Stephenie Meyer did not write Twilight to become the next Jane Austen. She wrote it because, like most authors, she had a great idea and wanted to share it with the world. So maybe her vocabulary wasn't perfect, but whose is? The Twilight series is a love story, so of course the prose is going to become a little flowery. Guess what? It doesn't matter. Books are not made to be divine works tweaked to perfection and set up on a pedestal. They are written to feed our imaginations and entertain us, and so they are written on a human level.
(By the way, for an example of Meyer's work that doesn't include the colorful vocabulary used in the Twilight series, I'd pick up The Host. It's a great book, with wonderfully realistic characters that is, despite the love story, intelligent and interesting.)