microATX motherboards are dying out - with the ability to omit drive bays entirely and just mount SSDs to the back of a motherboard tray, full ATX cases can get reasonably small now, so there's less demand.
It turns out PCI express slots are either something people want 3+ of (like a streamer setup of gpu, capture card, high end network card) or 1 of (GPU only) or none (business PC, devs happy with Intel or APU graphics). So really the only tradeoff the largest chunk of the market is making picking miniITX is that I haven't seen any with 4 RAM slots and a m.2 drive (though both features are available seperately on many motherboards). And even then the standard gamer setup is 2x8, while the amount of professionals for whom 64gb (2x32) is too little but 128gb is enough is currently a small market also.
Case manufacturers are not so quick to abandon the size though, with the result that if you get a case that you like for size/features/airflow and it happens to be a microATX case, most of your motherboard options are going to be miniITX.
