Adding some extra thoughts here, I don't think Smash's control scheme is particularly
simple compared to other fighters. By the time you unpack all the aerials, multiple jumps, tilts vs. smashes, parries, dodging, short hops, pivots, dashing vs. walking, and grabs, there's a fair bit of complexity in the controls that take getting used to. My sister still panics when she's knocked offstage and will do multiple Side Specials before she finally succeeds in getting an Up Special out (it pisses her off when I call her out on it every single time it happens), which is really annoying when
![Greninja :ultgreninja: :ultgreninja:](data:image/gif;base64,R0lGODlhAQABAIAAAAAAAP///yH5BAEAAAAALAAAAAABAAEAAAIBRAA7)
is her fave and I have to hear Shadow Sneak over and over. And I'm pretty sure she never uses her double jump.
Instead of simplicity, I think what most people here are appreciating is actually Smash's
uniformity. Every character fills out the same move list (1 jab, 3 tilts, 3 smashes, 4 specials, 5 aerials, a grab and 4 throws) and they all fill that same list out with their own unique moves (there is also the convention that only Specials can be a command grab). This makes it easier to pick up because once you've got the fundamentals down with one character, you can easily transfer that to any other character in the game. You'll have to learn the new character's moves, but you are still working off the same move list (I hope that makes sense).
Breaking uniformity disrupts that. Take parries for example. Sakurai made it a global change to replace perfect shielding, which kept the game uniform. Now imagine an alternate timeline where the change wasn't uniform. Mario can parry, but Luigi has to use the old perfect shielding. Peach can't do either, but Bowser can do both. Now you have to remember which of the 70+ characters is able to do which subset of the defensive options. Is it possible to remember that? Yes (ask any Pokemon player). Is it more difficult and a pain in the *** to do so? Also yes.