[asdff]

With this many pis, what's stopping you from buying an old workstation that can run all of those systems concurrently?

[solarkraft]

Management simplicity, probably. There are many pre-made images you can just flash on an SD card and have run perfectly. Computationally you could very likely run all of the services on a single Pi just as well.

[jeffdubin]

Most of these live in different locations, or are portable.