i feel kinda dumb for only realizing this recently, but it seems the amount of hardware available for every event correlates to how popular the event is, and how easy the puzzle is to design. it's basic supply/demand, i think.
3x3: so many options. you could probably find a usable cube in a dumpster.
2x2, pyraminx, skewb: less plentiful, but still plenty of good options.
4x4, 5x5, 7x7, megaminx: only a few good options, but all of them have their strengths & weaknesses. it's all up to personal preference.
6x6, square-1: a few options, but there's clearly only 1 best option. if you're at a high competitive level, you must use these.
clock: one. one option. don't even think about using anything else.