It took many years for Spectre and Meltdown to be discovered, and that was for CPUs affordable for individuals.

How many security researchers are even familiar enough with the concept of a mainframe to consider looking for an LPAR breakout, let alone have access to the necessary hardware?

>It took many years for Spectre and Meltdown to be discovered, and that was for CPUs affordable for individuals.

Both of these were anticipated in security papers dating back to the 1980s. It wasn't practical to use those types of exploits on 10mhz VAXen.

Also consider: anyone with access to a mainframe will never ever get approval to try to hack it because companies that have mainframes will never want the risk of accidentally breaking the host in some way.