I've been a Linux user since 1992 and I think I can enlighten a little on this.

I would guess in most cases in these drivers, there's no advantage whatsoever in using their own queues versus unbound workqueues. I think it's a simple matter of many drivers are made by taking an existing driver for a card that operates similarly, pull out the hardware-specific code, place your own hardware-specific code in then make changes as needed (I know in the 1990s and 2000s at least this was very common for network drivers and storage drivers at least). Per Google, unbound workqueues were added around 2013, at which point you had a bit over 20 years of existing drivers.