Because you're the one taking offensive to it.
I don't find it offensive, I find it stupid that someone is trying to summarise a book (any book) in about ten words or so as evidence that said book is crap. If you want elaboration, I think Pride and Prejudice is funny, I think the main characters spark against one another with the sort of dialogue that has influenced countless romantic exchanges ever since, and Mr Darcy is one of the best anti-heros in all literature. it's a book that still has a lot to say about human relationships. I can't really give a thesis on these things. What I do know is that it's on a different planet to Twilight.
"Excuse me Miss, do you like pineapple?"
Fine then, I will go into a deep description of why Pride & Prejudice sucked.
There was no depth to the story line. Most of the book consisted of surface level romance with very little character development for anyone other than Elizabeth, Darcy and Wickham.
The characters were either pompous or annoying, with the exception of Mr Bennet and Mr Collins, who provide some slight enjoyment of small sections of the book.
The plot is so remarkably simple that you could look at the front cover and pretty much predict the ending.
And finally, there is more dialogue in that book than there is clichés (which is hard to say). It leaves a complete black and white world of blandness until the one chapter where there is a bit of description of Mr Darcys house.
I suppose, taking that into account, Twilight is a little better. At least it attempts to engage the audience in another way other than female ramblings about different types of men.
I find Jane Austen to be an incredibly dull writer, though I admit I don't like books of that genre and era, for the most part. The only exception is probably The Woman in White by Wilkie Collins. That book is just great!
Apparently a lot of people didn't get the deep undercurrent of social commentary undercutting Pride & Prejudice. Which, if you don't know much about the era it was written in, is pretty easy to do. If you think it's "just surface level romance" though, then you've missed the point. Virtually every development in the novel is a backhanded commentary on some social convention or other of Austen's era.
All that said, I can understand not finding that enjoyable - I personally prefer the miniseries as well. But to claim that it's not good, or to insult it by mentioning it in the same breath as Meyer's work - is just shockingly ignorant, unless you'd care to demonstrate the same depth of social commentary in Twilight (which is highly unlikely).
I don't see much in common between the two, other than the central love triangle, and that's a pretty common plot element. The characterizations, themes, dialogue and morals are light years apart.
Of course, which is why it helps that Austen's writing and characterisation are quite astute.