MK is actually better than Sonic on this stage.
Anyways, reason #1 is because it's too big for a 1v1 map. It's more suited for 2v2 or free-for-alls.
#2 There's a couple of hard circles on the stage that fast characters can use to literally run circles away from their opponents and avoid all combat. We obviously don't want that, right? Since there's a dominant strategy to the stage, character choice is saturated to only the ones that can perform the strategy, which would be just Metaknight and Sonic. Nobody else can compete with those two characters.
#3 The cave at the bottom (although fun) makes getting killed very difficult. Enemies will bounce off the ceiling and get slammed to the ground again, or will get hit against a wall and tech the wall itself, negating any knockback on the spot. This is bad because getting 3 kills can take forever, and matches will always run the full timer.
#4 There are certain parts of the stage where characters who can glide can lock themselves into and be barely reachable. They can literally glide against a wall aiming downward but not descending. This is clearly stalling and we don't want to promote that capability.
#5 And probably the weakest reason of them all is that there are permanent walls in this stage which will promote wall infinites, such as King Dedede's chain grab. I say this is weak because King Dedede is NOT viable in this stage, and because nobody but Smashers will care if there is a wall in their stage.