The special theory of relativity is incorporated into the Quantum theoretical framework. The relativistic quantum mechanics (RQM) gave the first insight into the existence of antiparticles-the pair productions, the decay rates, spin magnetic moment etc. What you are asking is, perhaps, why General theory of Relativity (Gravitation) is incompatible with the Quantum mechanics, right?
The problem is the quantisation of the highly non-linear gravitational field equations and to make meaningful operators out of them. One needs to find a way to have quantized space-time structure and then the smooth manifolds over which the general relativistic equations are defined cease to exist. In the general relativistic description the space-time is a dynamical background (field energy content changes the nature of spacetime, sort of a back-effect) affecting the fields while in the RQM/QFT its a static stage on which various fields/operators live and interact.
Then there is a problem of carrying over the parameter time in quantum mechanics to general relativity. The notion of absoluteness of time (non-local) leads to the stationary states as solutions of Schroedinger's equations. How to arrive at an unambiguous definition of time (local) in a quantized version of gravity could be one problem.
There could be many other deeper layers to the problem of compatibility of the two and those probably will be discussed here by some physicist working with quantum fields on a curved space-time.