As Steve mentioned, one huge advantage of the 8051 architecture is its uniquity - it seems like everyone and his dog makes 8051 variants!
You have not only 2nd sources but 10th, 11th, 12th, ... ad infinitum?

Also, there are many IP vendors offering 8051 "cores" for implementation in ASICs, FPGAs, etc, etc - which could be why the 8051 seems to be the core of choice for people needing to embed a processor within their custom device?