The reason given is it's getting difficult to find COBOL programmers and the platform isn't as agile as other platforms. I have to admit that IBM is working to make the platform more agile and you can do more than run COBOL - you can run Java and Linux on Z. I've been in IBM training sessions where I've been running Linux on Z and then running Docker on top of that and unless you had told me otherwise the experience was normal: you're running bash, running Docker and everything runs as you'd expect.
BUT - the problem is the core of our system is written in COBOL. There's still CICS, JCL, Copybooks and DB/2 and those who can work with those technologies are now within 10 years of retiring and the new generation doesn't know them or have much of a desire to learn them. Heck, I'm part of the first generation of PC developers from the mid-1980's and I've never done any mainframe development!
The thinking then is we'd better jump off before all these people retire and you end up having to pay $300+/hr for resources capable of working with these legacy technologies. Pay now and buy yourself a new technology set and new capabilities or pay later and just keep the system running.