It's because Blastoise got access to Shell Smash in Sword/Shield. Its natural bulk allowed it to set up Shell Smash on many things without even needing to worry about the defense drop. At +2 speed, it outsped the entire unboosted metagame and only a few prevalent Scarf users were able to outspeed it at that point such as Greninja. Its strong coverage options let it break through the vast majority of the metagame.