And the reason they are incompatible is not obvious at first sight, but the basic idea is simply that quantum mechanics is based on the idea that things can flutter, things fluctuate. Even if they are as in the lowest possible state of energy, even if they're resting on the table, they're still vibrating. And, if you try to combine that with general relativity, you run into big problems, because general relativity wants to model the universe as this very smooth, placid, varying space-time geometrical shape, and that's at odds with the frenetic flutter and jitter of quantum mechanics. That's why they turn out to be incompatible. So for many years we've been faced with this terrible problem that we have theories that work very well in certain domains, but they are incompatible with one another.
String theory comes along, and solves that problem.
String theory comes along and says, do not think about elementary particles as being little points. Think about them as being little loops. And when you do that, it turns out that you get exact meshing of gravity and quantum mechanics. That small, little change winds up doing that. In essence, replacing a point by a loop spreads out the jitter, and spreads out the jitter in just the right way to make it compatible with general relativity.