There's plenty of space for an MATX motherboard. Just need to find an alternative place to mount the PSU.
I can see why it isn't a standard option though; the alternative options for PSU mounting aren't great. It could go on the front panel, against the glass. But then the PSU is intaking heat from the GPU and the PSU cables are visible through the front glass. Or it could go up top, reducing radiator compatibility. Might also be able to squeeze it in at the bottom, but with reduced GPU clearance.
For most users, the standard layout with an ITX board is going to be better TBH. Swapping the front glass out for mesh and moving the PSU to the front would probably be the best way to deliver MATX compatibility.