A lot of mainframe users never needed a mainframe, they just didn't want to get fired for not buying IBM. Other users need some sort of mainframe reliability, but that's also achievable on a distributed system running on unreliable PCs.
There are some users whose use is genuinely deeply entwined with features mainframes provide, but those are dying out.
I don't buy this. Many of our critical systems are on PC architectures. Mainframes don't have some magic sauce, well designed distributed architectures should offer enough reliability.
Edit: reading comprehension fail, please ignore my comment :-)
Why waste your money on a single rack-mounted PC when you can buy 40 cheap cellphones running Android and network them together. That'll probably provide greater reliability.
The obvious answer is the same as why some use mainframes over PCs. You can't easily convert all workloads running on a rack-mounted PC to a network of cellphones. Similarly, you can't easily convert all programs running on mainframes to running on PCs.
Which is what I was alluding to with the last paragraph in my upthread comment. There are mainframe use-cases that are genuinely entwined with those hardware platforms. A mainframe isn't just a fridge-sized PC.
https://it.toolbox.com/blogs/shayne-nelson/the-60s-ibm-the-s...
By the 1990s, "big iron" referred not only to mainframe (S360 descendants -- now S390 / zOS), but big Unix vendors: Sun (now Oracle), HP, Data General, SGI, etc.
Some of these still survive in some form:
https://en.wikipedia.org/wiki/Mainframe_computer#Current_mar...
In mainframe hardware, IBM doesn't have much direct competitors. In mainframe software, there are many but most of the are much smaller than IBM, so it is often more symbiotic than competitive relationship.
However, IBM has dominated this market for as long as I can remember and today I believe they are the only ones making mainframe computers.