One tends to use the tools you know.
If you (or your predecessor) had the foresight to pick a processor vendor that covers a large spectrum of potential performance needs the shortest path (inertia) is to stay with the same vendor. Not having to buy new tools, skipping the learning curve for a new debugger, and knowing how documentation is organized count for a lot.
I would have to agree with Walter. Why spend all that extra timne learning how to use another vendors toolchain when you have already spent the time with the one you learned.
Odds are good that the mfgr you have chosen has enough micros to keep you satisfied and on the front edge of technology or else they wouldn't be much of an MCU mfgr.
