The x86 instruction set has evolved over time and it isn't that uncommon for modern apps (especially games and media apps, but also sometimes just run-of-the-mill desktop apps) to assume you have SSE extentions, at a minimum, and not offer a fallback if you don't. That makes a super-fast 386 very impractical for Joe Q Public running Windows and thus makes it not nearly worth the engineering effort required to produce such a chip.
They could possibly get away with a super fast 386 in the Open Source world where you can just recompile everything to your CPU's specs, but if that's the only market you want to hit and you're going to force recompiles, why bother with half-assed old x86 compatibility anyway? In that situation just make whatever instruction set you want, create a backend for the open source compilers and just do your best to keep things source code compatible with the x86 (eg. make it little endian, etc).