Arm Ache and finger prints
More like they just don't want to redesign the iMac to work for that. Surface Studio style setup would be perfect for it.
The new Mac Pro seems to make the same mistakes the previous ones did. Few people actually have any need for server class processors, gobs of ECC RAM or VRAM but would like something more powerful than the regular iMac.
If they can cram this all into a small case like the iMac, why not just release a small box with all these goodies inside so you can upgrade it when needed and use it with any display you prefer?