I'm sure SI is aware of it, but I would guess:
1. They're waiting to actually see FFP come into effect to see exactly how it is enforced, so as to know how to properly code it in Football Manager...
or (more likely)
2. Considering how money can be shuffled around so as to sidestep the letter of the FFP law as it is currently written (see: Etihad Stadium sponsorship deal), and considering how such boardroom-level financial affairs are traditionally omitted from the game (i.e. "this is Football Manager, not Football Owner"), they'll leave most/all of FFP out of the game.