How did the CX85 work? If I recall, you had to have software that recognized it (CX85 driver). This means that for the majority of software out there, it would not do anything except take up table space (making for a nice paper weight). Kinda reminds me of how the XEP80 worked as well.
So if I had my choice between a joystick or an SIO peripheral like this one, I would think SIO would have been a better move since it could of at least pre-loaded its own handler, something that a joystick device can not do. However even better still, is a device that can utilize what has already been built-in to the system OS, that being the keyboard interface via POKEY. So that is what the original TransKey (1990), the AKI (aka: KRH), and the new 2015 TransKey-II all have taken adavantage of in their designs. As such they require no drivers, and are completely transparent to any program or game ever written for the Atari 8.
And getting a mouse to work through this interface, once again means absolutely no driver required, as would be the case for using an ST mouse. So basically this stuff just works right out of the box.
"As I remember USB is direct successor of SIO. So, why PS/2?"
USB may indeed be a descendant of SIO, but that doesn't mean that they are compatible. Which in fact they are not, neither from a hardware or a software aspect. Implementing a USB 'HOST' is no trivial matter, and not something I presently feel capable of doing. So that is why I took the much easier to implement PS/2 approach on this go around. Ironically it is much, much easier to implement a USB client, and there are quite a few embedded processors that have this technology built-in. As a client, you are the mouse or the keyboard. But unfortunately for what we are doing, we need to read the mouse or the keyboard, so that requires Hosting.