-big post-
The reason the ZiS is slower is because it is harder to decrew and doubles as artillery if you pay for it. A PaK for the same price comes with 4 men and can only harm tanks. A ZiS comes with 6 men and can shell said PaK out of existence if it wanted to. You can use it to shell enemy MG's or capping squads, which you cannot do with a PaK.
The reason the Ostheer Mortar fires faster and more accurately is because it's fighting against things like Conscripts and Maxims which are very mobile and have large squads. The Soviet Mortar is slower and less accurate because it's fighting a more campy enemy with smaller squads, but it gains "precision strike" to force-retreat or kill enemy support teams if needed.