They didn't do it because the K-3 is coming soon with a 24MP sensor, 21-point AF, Prime III image processor, and a new flash system. The K-5II is just filling a gap. The K-3 had to be under development and on the product road map prior to Ricoh buying the company. I would be shocked if work on the K-3 did not begin the day after the K-5 was released.
Prime M is for cameras that depend heavily on CDAF. CDAF takes up a good bit of processing power while PDAF has its own sensor and processor. Prime M is just optimized for CDAF and HD video at the expense of still image processing power.