Ok, but 20 is closer to 24 than 30, and is also divisible by 60 so is a viable target.
So you obviously prefer 20fps to 30fps then? Given the huge increase in fidelity it implies, after all.
60 is not divisible by 24.
The standard 30fps was choose because 60 (the most common refresh rate in NTSC TVs) where divisible by 30 (the TV have to duplicate all frames to reach the refresh rate).
That in NTSC format of course.
In PAL formats (50Hz) it was 25fps (that is where come the 24.9fps from movies)... in fact for NTSC we should have movies in 29.9fps and not 24.9fps except cinema uses another format so they standardized 24.9fps.
Technically speaking for not have any issue the games should have different framerates for:
NTSC (and others 60 base formats) TVs: 30, 60, 120, etc.
PAL TVs: 25, 40, 100, etc.
That is why PAL users suffered a lot in the past because they have to accept games in 30/60fps in 50Hz TVs when obvious these games should have run in 25/50fps in these TVs.
PS. Today TVs doesn't have different refresh rate standards so that NTSC/PAL issue is a thing of the past.