Consider the time. A 'server' usually meant 'shared file and print', not so much applications (or at least limited ones). And the disks were a couple of orders of magnitude slower than they are now. So you had a lot less overhead and could heavily optimize for smaller numbers of use cases (which made Novell very successful for a good while, tho not on z80). Also, networks were a lot slower. Ethernet had just been introduced and hadn't taken over the world. Most nets were some sort of serial network (< ~1Mbps) or maybe Arcnet (~2.5Mbps). So, yeah...there were Z80 servers. MP/M was a common operating system for them, but as we see it wasn't the only one.