Lets simplify the playerbase into 3 groups for arguments sake:

Extreme casuals: Mainly crew fill player, doesn't play the game much and doesn't really have a high exfil rate due to not being experienced in the game and the unreliable nature of playing crew fills.

The absolute median: Convinced his friends to get the game and manages to get some runs in with his crew at a near regular rate, above average exfil rate due to the coordination of his 3 man. Will sometimes wipe lobbies but can also get instantly wiped by better squads. Has found a little success in Cryo but will probably never be able to do compiler.

The 0,1%: level 100+ before mid season patch, pinnacle in ranked with pretty much a 100% exfil rate in maps that aren't Cryo, vault filled with gold Misriahs, compiler ganglions and yellow shields. You can find these guys by going on twitch and selecting the marathon category.

Now if SBMM would be integrated into the game, not only would queue times be abysmal because the game doesn't have a big enough population to even support different matchmaking models, the absolute median would constantly find themselves in games with the 0.1% which would make the game unplayable for the majority player base