While Blissey is a big part of special attackers being way less viable than physical attackers, it's not the ONLY reason. Swords Dance has MUCH better distribution than Nasty Plot, and while physical sweepers get Dragon Dance, special sweepers get...Calm Mind. That means that special sweepers don't become nearly as threatening as physical sweepers do after one turn to set up. On top of that, the only special priority move is Vacuum Wave, meaning special sweepers can't pull any of Scizor's Swords Danced Bullet Punch shenanigans. Finally, look at the most powerful physical moves compared to the most powerful special moves. Physical sweepers get Close Combat, Outrage, Flare Blitz, Earthquake, and Brave Bird. Special sweepers get Draco Meteor, Overheat, Fire Blast, Hydro Pump, Focus Blast, and then you're down to stuff like Surf and Thunderbolt (and there's stuff with really poor distribution like Eruption and Water Spout). The special moves either have less base power or some serious drawback; Focus Blast has 70% accuracy, Draco Meteor and Overheat cut special attack by two levels (and they only have 90% accuracy). The drawbacks of Close Combat and Outrage don't prevent things from taking out a dedicated physical wall; Lucario can set up Swords Dance and 2HKO Skarmory with Close Combat. In comparison, if Azelf DOES manage to get to +2 and tries to take on Blissey with Focus Blast, it's facing a 51% chance of a miss one of those two times. The only specially based sweeper who really had the bulk and the speed to get up to +2 reliably was Manaphy, and even with its godawful movepool, it was declared Uber. Physical sweepers can either get up to +2 Atk or +1 Atk/+1 Speed all the time, because stuff like Tyranitar, Dragonite, Gyarados, and Scizor have both the bulk and the moves to get away with it. So while Blissey may be the nail in the coffin for any Pokemon that attempts to sweep from the special side, the truth is that specially-based sweepers have a lot of disadvantages to begin with; they don't boost their speed, their moves aren't as powerful, and most of them can't get near sweeping range in one turn, even against a weakened team. What Blissey does is makes it so that we don't even have to worry about prediction or choosing from a handful of counters that the Pokemon could be designed to beat whenever something threatens to sweep from the special side (which we obviously do from the physical side; sure, I can switch in Rotom-A against Lucario, but it will Crunch?). We just pick the same pink blob against almost everything.