The same reason Blaziken was tiered as Uber in previous generations: Speed Boost has the potential to get out of control on something with as much offensive presence as it has, and unlike Sharpedo, Blaziken doesn't even have to give up its ability if it goes mega to secure even more power. It may not be good in Ubers/AG/National Dex AG, but you can at least use it there if you want to use it at all.
