A lesbian focused blog going by the name of Autostraddle attempted to delve into it and figure why exactly this is the case. It's actually a pretty entertaining read, especially if you share my complete confusion on Twilight and Rainbow Dash are apparently soul mates in half the shipping fics out there.
Anyway, check the article out here!
And in the comments below: What are your views on all the girls on the show having romantic interests in each other? Are you a fan of shipping in general?