Part of the reason is for the past 100 years we've been teasing out all the details of these theories and verifying them. We've only recently discovered where the incompatibility of these theories present problems - and now we're tackling them. What I'm really saying is there hasn't been a whole lot of people trying to bring together QM and GR, not to the level that's going on at present. For those who had been trying to reconcile the two theories they were working with theories in progress, theories who's impacts were just being understood. As another commenter mentioned these things take time to bake!