Because if Tim Miller is developing an origin story for Kitty, then they might want to recast, and have a younger actress play the character before/during her schooling at Xavier’s.
Obviously the most obvious choice would be Ellen Page, who already played Kitty twice in live-action.
Page has already expressed her interest in returning to her mutant role, although she hadn’t been approached by anyone.
It’s all a mystery, although the hardcore fans would no doubt like to see Page get a meatier role in a third X-Men movie.