Because EVs in all stats aren’t good (usually).
For example, a sweeper needs 252 Spe and 252 Atk/SpAtk to sweep. Sometimes this isn’t true, but most of the time it is. When was the last time there was a defensive and mixed Kartana? Never, because 252 Atk and 252 Spe are way better than a spread between all of its stats.
On the other hand, a wall/staller/tank shouldn’t have EVs invested in every stat, because, well, they aren’t meant to attack, and their attacks wouldn’t be 1 physical and 1 special (if there even is 2 moves on one of these). Speed is super unnecessary because usually these Pokémon are slow, and they don’t need to go first. It’s much better to do 252 HP, 128 SpDef, and 128 Def on these Pokémon. Yes, I know this isn’t maxing 2 stats out, but it’s the best option for a wall/staller/tank in my opinion.
Note that this answer isn’t for LC. In LC, mixed attackers will sometimes have EVs in all stats, due to the fact that EVs work differently.
Hope this helps! :)