Because it's not accurate.

I knew the article struck me as incorrect in it's assumptions.  I could outline them all for you - and I was planning to.  Then one of my favorite sites - getreligion.org - posted their own critique on the article, which tracks pretty well with what I was going to say - and saves me the trouble of saying it.  Summed up - there are lots of things credited in the first article to Mormonism that are hardly uniquely LDS.  It would be more accurate to attribute these things more broadly than to assume that just because the Twilight author is Mormon, the ideas that she's depicting are specifically Mormon also.

As an aside, I haven't read or seen any of these, and don't have any intention of doing so.  Are you a Twilight fan or not?  Why or why not?