Mainframes are very expensive. You can buy mainframe with very fast CPU and RAM interconnect and scale it by buying more hardware. Or you can spend 100x less and buy a number of server blades. Interconnect will be very slow, so you can't just run some kind of abstracted OS, you need to run separate OS on every server blade, you need to design your software with that slow interconnect in mind. But in the end it's still 100x cheaper and it's worth it.

Also mainframes have growth limit. You can buy very powerful ones, but you can't buy one that's as powerful as entire datacenter of server blades.

amen, and those principles still serve us well today. software developers are still as bad at writing code, maybe worse.