I think it can be summarised by features and aesthetics.
Features:
SLI and CF are still around and there are some who see great benefit to them. Some games support them in DX12 very well, others not so well. It is an area that needs further direct input from the developers. But some features like double M.2 slots or 6x Sata or similar are equally important to some people and tend to only be available on larger boards. I prefer the larger boards for the wider range of features available. As mentioned above, the better features like excellent VRM solutions as well as
Aesthetic
I've owned some large cases (TJ07 & 900D) so I do understand the real estate problem some people describe. I use the larger cases for the water cooling space. A mini ITX or mATX board in a large tower looks odd especially when it is dwarfed by a full length gpu and two 480mm radiators. Is the board required to be ATX, no. Do I require the space, yes because I need the extra space for silent fan profiles. Do I like the look of a mini ITX board in a full tower case, no. So that is why I will go for ATX as long as I am able to.
I do love the look of some of the ITX boards however and a mini powerhouse under my tv is something I'll do in the future for sure.