I think it’s also both the people making the tech decisions and the people implementing them simply don’t want to work on mainframes. I would 100% of the time prefer to work with an on-prem dc or colo using commodity hardware and OSS to mange it.
I get my pick of a billion different hardware vendors, they’re all interchangeable and interoperable, every problem I can possibly dream up has been solved a hundred times before. The skills are commonplace and transferable so hiring doesn’t mean convincing some poor desperate grad to get trained in the company script, hires will actually want the skills, and you can find senior people.
Mainframes are still the best option for systems that cannot fail. Even business like FedEx is not as critical as some types of business I've seen ran through IBM.
A good example of where these systems come into their own is payment/ACH/wire processing. The consequences of these networks going down are so severe that it is worth it to construct an entire facility with the computer systems and business in mind from the very beginning.
Today, mainframes are more for the type of business that are finding the need to pour the literal foundations for their own datacenters. If you are even considering cloud as a viable path, then this kind of stuff is certainly not for you.
It's probably this. I've spent time trying to advocate for the benefits of these systems, but its like shooting a super soaker into the sun.