I use a z170 stinger in an itx, board has been great but it seems no one even owns one so there isn’t much community to discuss things with. I speculate sales, particularly on better itx boards that cost more are low. You can probably glean that from Amazon reviews. I have one of those Asrock Z490 itx tb3 boards and even if you combine the number of reviews it was with a lower model it’s still only a mere fraction of a single model of oje of their standard ATX boards.
Its probably due to factors like cooling, SFF PSUs which may be needed certain sized graphics cards. It becomes more and more specialized. I know when I did the EVGA z170 stinger build I had to get a certain height cooler, use a Zotac twin edge video card due to length, use relatively short ram (so no super tall heat spreaders).