Because they're mostly dreadful.

Vampires are 'in' again and so Meyer's worldview has commercial worth. It's pretty cynical stuff overall.

As a 'love story' it's one-dimensional - just because it's aimed at young'ns doesn't mean it can't be otherwise, unless we're actively seeking to condescend to the young.

Absolutely nothing is made of Bella's essential emotional transience, her moral fickleness or the fact she invests all trust and faith in someone at the expense of actually living a life.

I can see why that sort of thing - the uncritical stance on many would-be important issues - has a certain appeal to a certain sort of mindset, though...