Because of usage. Showdown works with usage rates; if people use, say, Sunkern enough in OU, it'll rise to OU. This doesn't mean Sunkern is viable at all, but people used it enough for it to go to OU.why is blissey still OU even after bloodmoon ursaluna qnd gliscor have been banned?
Blissey got to OU by usage, and it seems people are using it enough to do so for two months straight; it's still a good special wall, even after the Toxic loss and recover nerf. People found use in it, I guess.
Tldr; blissey was used enough, OU doesn't nessecarily mean viability, see gen 4 OU dusknoir