This has nothing to do with airworthiness, purely because they are VFR only aeroplanes, and thus these instruments aren't required - only a compass, ASI, Altimeter and basic engine instruments.
The instruments fitted are entirely market driven, presumably with an emphasis from flying instructors who like them for teaching purposes, and to a lesser extent PPLs who want a "Get out of Gaol Card".
Also with micro/ultra light aeroplanes, there's always a weight penalty, and an AI is a heavy instrument, which is often the main reason for not fitting one. Most AIs also rely upon vacuum, and Rotax engines don't take a vacuum pump so far as I know.
G