I don't know why Smash players think each game is so distinct when there are so many basic mechanics people forget are the same. Every game in the series features percent based damage that correlates to the knockback of moves. The goal of the game is to knock your opponent off the stage and if you get knocked off, you must use jumps to get back. Things like l-canceling, wavedashing, ledge mechanics, and hitstun are just nitpicking at the little difference that the game have, but in reality they're pretty much the same.
So ignoring these minor details, here's why smash has a huge skill gap compared to many other games. The skill floor is low since the game is so intuitive. Knocking people off the stage is a simple goal made easier by the controls. You only need to know how to move, attack, and jump and you can compete.
On the other hand, the skill ceiling is amazingly high due to the highly dynamic and nuanced gameplay. Have a game based around knocking players off instead of getting damage means players can live indefinitely, yet die at any time, meaning player skill completely dictates how long you live. Knockback rising with damage means players have to consider how each move will effect the next and judge how the percent correlates with the knockback in order to properly follow up, meaning moves constantly change in purpose and reward. Speaking of following up, DI gives the defensive player some amount of control at any time and makes the offensive player always adjust their attacks to account for DI. The high amount and control over movement options (5 levels of walking, dashing, running, multiple jumps, and high aerial mobility) allows players to micro-manage actions to maximize rewards and the high amount of defensive options (shielding, rolling, airdodging, and spotdodging) means pressured players always have something to combat the situation. Even in highly disadvantagous situations like on the ledge and on the ground, players are given various options and free control over when they perform the action. Official tech like l/z-canceling, shield tilting, teching, and pivots attacks couple with glitches and exploits like DACUSing, wavedashing, and other ATs just add to the skill ceiling.
It's sad that instead of answering the question OP presenting, we just devolved into a Melee vs Brawl/Smash 4 debate just like EVERY OTHER CONVERSATION about Smash games. Please, we're better than this. The OP asked a genuinely interesting question. Let's respect that by keeping the "which game is better debate" out of it.