I think it's mainly that from 2006 to 2019, the iMac was the only mid-tier desktop that Apple sold. Today, you can get a Mac mini or MBP that's the same or faster than an iMac.
I did have a 2010 Mac mini server, don't remember how it compared to the non-server one other than having dual HDDs instead of the DVD drive. I put them in RAID-1.