It's not about difficulty, it's about balance. A big part of competitive rulesets is about removing randomness from the result wherever possible.

I get that a lot of stages are not viable due to how drastically they change the game's dynamic, but some criteria I have seen for deeming stages illegal seem backwards to me.

The entire point of having more than one or 2 legal stages is so that no single character is too OP on that specific stage without a chance to counter it, which is why competitors at tournaments get to alternate their stage picks between matches… so why is something like "Its upper blast zone is slightly smaller than Final Destination's, so it can't be legal" a good reason to ban a stage..? That is not a stage hazard or a randomly-generated event. If anything, Smashville's moving platforms are way more RNG than that and that is a legal stage, and people HAVE died to those platforms suddenly retracting in tournaments, and yet a slightly jagged stage corner or a slightly different blast zone are too much to handle, despite such factors actually promoting the use of different characters without breaking the game's meta..? I will have to agree with the attacked side on this one: current criteria for legal stages is too strict and arbitrary in more than one aspect that actually does the opposite of balancing the game by favoring character archetypes needlessly.