Because it doesn't have a good match up in ou. In gen seven, Blaziken had the option to brute force its way thru its counters via electric z crystal boosted thunderpunch, which would kill Toxapex, Slowbro and Tapu Fini in one shot. However, z crystals are no longer a thing in gen eight and as such, it can no longer kill it's counters with thunderpunch boosted by adamant nature, life orb and thunderpunch, altho it does maim them real hard but at the same time, they would also kill the fire chicken with their water type move. Not only that, Landorus Therian is also back and a bulky set can live a flare blitz after swords dance and intimidate and can kill Blaziken with one earthquake, even if it is uninvested
These struggles, as well as the fact that even though Blaziken has speed boost, it still has only base eighty speed, leaving it outspeed by many choice scarfers and some offensive threats even at + 1 speed, has led Blaziken drop to uu. In the council voting ban, Blaziken barely missed the banhammer but the playerbase wanted it banned and as such, a suspect test was made. Blaziken was banned after the voting stage and even though it is a big threat, it is still a big hit or miss Pokemon and ou players usually want Pokemon that can put in consistent performance
In short, the playerbase decided that Blaziken was too powerful for ss uu but is too much of a hit or miss to be consistently used in ou
Source: Experience and this https://www.smogon.com/forums/threads/np-ss-uu-stage-8-1-this-hearts-on-fire.3674927/
