There are alot of things to consider with adding firmware code.
The Pro and Plus may not have enough storage to store this new code, as they already contain additional stuff that the normal does not have.
It could also be limitation of heat and power, as again, the Pro and Plus generate more heat and consume more power as they have more stuff to do compared to the normal.
There is another thread comparing the heat generated and power consumption with the normal and Pro and they are significant.
There is alot more involved with a WiFi client compared to a WiFi AP.
Dumb AP just needs to broadcast SSID, accept anyone with correct auth, and assign IP first come first server retire after time out.
Client on the other hand needs to be able to scan for SSID, handshake and negotiate auth, request and renew IP, etc etc.
Alot more code and processing involved.