You have to imagine that the design work done for that was based on an assumption that Apple would stick with the trashcan form factor long term, and that the first gen unit would at least get regular bumps to newer CPU and GPU options.
Must have been a heartbreaker sitting on all that infrastructure in July 2019 when the cheese grater was announced and the newest trashcans were still selling for full price with a six-year-old processor.
Apple gathered a few people from the press to tell them the trashcan wasn't working out in April 2017, and that a redesign was coming. Everyone who cared about the Mac Pro knew then.
Regular server racks have not, though. 1U has been 1U for a very long time... Didn't matter if it was an IBM server, Dell, HP, or custom built. This is why we have standards.
These poor saps bet BIG on a non-standard, and seem to have discovered why we like standards after all.
For example IMGX uses Macs in its data center for image processing.
https://photos.imgix.com/racking-mac-pros
I remember reading somewhere the performance they got with CoreImage was worth the extra hardware cost.