Mini ITX was picked up because that was all there was for SFFs, but the selection is tiny compared to ATX/mATX. A serious SFF standard would drop ATX and have a single 24v DC coax input and rely on VRMs to translate voltages as needed on the board. Then you do away with the power supply in the case and use a cheap 24v brick to power it. It would drop all the legacy crap with those fiddly little headers for connecting front ports and LEDs, and stick an RJ45 port or two on the board to handle those connections to the case. That sort of thing.
But there's no one with authority to say "hey that's a good idea" - a board maker and a case maker might both recognize that dropping headers for RJ45 makes sense from an ease of use and cost perspective, but they can't implement that because unless all or at least most boards and cases support it you're just selling this weirdo product with an RJ45 that requires you make/buy some Frankenstein cable to connect it to the legacy headers on the other end.
What incentive does Dell have to create a mobo standard with DIYers in mind? Dell doesn't sell motherboards, they sell PCs.