Bootcamps saturating the market with webdevs armed with skills that barely last a year, when they could be training people with skills that would see them employed for a whole career.
It has nothing to do with bootcamps. If the mainframe knowledge was in sufficient demand, then they would be selling mainframe training too since people would be wanting to buy it.
A large part of that is probably due to American management practices, where the preference is to skimp on maintenance until they find themselves in a crisis. They don't want to pay for what they need, they want to pay for whatever is cheap.
Maybe, but it may also be true that people simply aren't aware that mainframes exist or these jobs exist.