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Learning web, mobile phone or PC development is rather cheap and easy.

The hardware costs for even a beat up old mainframe are terribly high, and IBM frequently gets upset about the Hercules emulator project (which lets you emulate System/370, ESA/390 and the zSeries). Without Hercules, this makes the entry costs to learn mainframe architecture and application development out of the reach of all but the most wealthy hobbyists.

No college I've attended since the 80s has had any mainframe available for student use. I think IBM and the rest of the ghosts of the mainframe industry shot themselves in the foot making them less accessible to learning.