Its called chromatic because the layout is chromatic rather than random. It would be very hard to learn if it were random.
Even on a traditional bandoneon which is diatonic and not chromatic there is some system to it. (There are several different layouts in fact.) I dont know of any instrument where buttons are just laid out randomly. The only case where I saw something that looked quite random was the placement of chin switches on the Pigini P39 or C39 bass accordion... until I asked for these switches to be in the same order as the regular switches, which may have woken them up. But note buttons I have never seen arranged randomly.
If you dont know what the chromatic layout is:
https://en.wikipedia.org/wiki/Chromatic_button_accordion