From the posting: "This extra performance comes at a cost: it consumes more time and more memory to JIT compile using LLVM than using Mono's built-in JIT, so it is not a solution for everyone."

So, faster code but more memory usage and longer JIT time seems to be the reason why mono didn't just switch over to LLVM.

My bet is that they'd switch if LLVM supported the same features and architectures as their own, but just with better results.