I bet they figured that Kirsten would've stayed the whole run of the series, so have her be the hero's girl for all of it instead of getting a Gwen for 1-2 movies then getting an MJ (and a convoluted storyline on top of it, I'm sure). However, since MJ is punching out after this one, they'll reverse the roles. Have MJ die or disappear with Gwen as the new girl of Spidey.
Makes perfect Hollywood sense. (Not that I agree with it)