It is actually obsolete from firmware point of view, and in spite of the efforts reverse engineering it didn’t get right most of the architecture. In the other hand there are other mainframe emulation projects sold by IBM which are capable to scale quite good, but in one hand it was the fear of killing the mainframe (e.g. offering emulated mainframe infrastructure from a cloud). In the other hand it was y he fear of a weak architecture reselling an emulated mainframe without all the warranties that the actual hardware and firmware provides made them sell those only licensed as development kits.

Finally everything is a business decision.