I think part of it is that for ports up to 10gbit/sec there aren't really as many advantages to using type C on a desktop as on a laptop.
Most desktop computers have dedicated graphics or at least the ability to have dedicated graphics and don't have the input connectors to feed video from the GPU to the USB mux. Most desktop computers can't run off of USB power. Most desktop computers aren't installed in environments where a single cable running to a desktop dock is desirable. Most desktop computers aren't space constrained.
Also adapters that convert a type A port to type C are tiny, they stick out about as far as a mouse dongle making the whole assembly not much larger than a normal A cable. Cables with one A end and one C end are everywhere. Dongles just aren't as inconvenient in a desktop context where you plug them in and leave them.
If you don't actually need the extra data lanes in the connector, there's not really much advantage to using it and for a desktop that is likely to be replacing an existing one with a lot of A connectors plugged in that's a hard push.
IMO laptops should have one or two A ports and as many C ports as they can fit, but desktops I'm not against leaning towards A with just as many C ports as they need to support their USB3.2/4/TB capabilities.
I've been looking for a powered desktop USB hub with both A and C ports and there just doesn't seem to be any reasonable products out there. It's all laptop hub and docs that require that connect to a C port via a 4" cable.
And usually its on the case and not at the back of the MoBo. Mine does have a single one, but on the MoBo since I made sure that I had at least one for my capture card.