Well, first off, their endurance is currently not even good enough to replace DRAM, let alone replacing critical path transistors. They also take significantly

Well, first off, their endurance is currently not even good enough to replace DRAM, let alone replacing critical path transistors. They also take significantly more power to switch and are much slower. They also tend to be highly unreliable (they even have variable write time!) and hard to manufacture. Really, the only useful advantage they do have is their theoretically incredible density, but even most of this comes from the possibility of 3D stacking (also possible with transistors) and from the possibility of a crossbar memory, but crossbars have epic sneak path issues.