Not sure why she can't find the time for both like what Jennifer Lawrence is doing with X-men and The Hunger Games.
Lionsgate and Fox coordinated shooting schedules, with Fox pushing the shoot for DOFP
back a bit, to make sure Jennifer Lawrence could do both. Sony is looking to crank out Spider-Man films at a very fast pace, leaving little wiggle room for coordination with another franchise.
Of course that may just be a convenient excuse for any recasting. Who knows.