I'm no pro in this field, but for one thing, quantum mechanics doesn't deal with gravity. And GR is smooth and continuous, but QM is discrete. I guess mainly, as one approaches the Planck scale, Heisenberg Uncertainty becomes more and more applicable, and GR doesn't deal with that. At Planck scales the very geometry of space becomes jittery and uncertain*, and no one has figured out how to define the metric in such an environment; hence, GR is left behind.
Originally Posted by Aethelwulf
* I'm not sure what the conjugate variable is in this situation.
Everyone is entitled to his own opinion, but not his own facts.