A lot of people need Mac servers (eg. for building and testing macOS and iOS apps). Right now, the best computer for that use case is an iMac. iMacs are really inconvenient as servers.

The Mac mini is a perfect server: needs little power, it's small and stackable, easy to replace if something fails, doesn't have unnecessary components like big graphic cards or screen.

I'd love to buy a couple Mac minis as build servers and for running unit tests, but the current version is too slow and way to expensive.