Look - Smash's 3-4 stock matches allow Smash to really become the unique fighting game it is. Smash is fundamentally different than other fighters, and has so many map-based and platforming mechanics in its fighting system, that we have to let players interact and strategize around these elements during a match. Reducing match time reduces the amount of mechanics within's Smash system that flourish and have an effect on the fight - the mechanics that make it unique and interesting fighter to begin with.
There is stage control, respawning mechanics, gimping, spatial-based mindgames, and more There is a huge stage-centric aspect to Smash that other competitive fighters don't have, and it is de-emphasised in short matches because fights become more about the aggressive-skillset, and less about the strategic, tactical, and creative skillset. Certain strategies, playstyles, creative styles, and tactics won't come to life in super short matches.
Quick matches will get us rock-em-sock-em matches where players are more aggressive, but unique playstyles and pace variation won't be as useful a skillset to master. Playing uniquely and formulating unique and skill-based tactical ways to defeat their opponent will likely not help you as much - instead, fights will be about aggressive dexterity, much like the more traditional competitive fighting game.