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Really? wtf. But i never knew that so that might be handy info to use ingame.
It's because of the way the game works. In order for a tank to fire, it has to rotate its entire hull and turret towards the enemy instead of just the turret (might be an engine limitation). The thing is, the StuGs rotate much faster than P4's and don't have a turret so they get on target quicker. Another reason the P4 is utter garbage.
 
so there's no advantage to have a turret in tank vs tank combat.
There is one upside in larger engagements. The turreted tank will face its strong armor towards the greatest threat while engaging different targets, whereas an SPG must face its armor towards its target regardless of the largest threat, opening itself up to side shots.
 
Also, I'm pretty sure SPGs with destroyed tracks can rotate 360 degrees anyways in SD; so there's no advantage to have a turret in tank vs tank combat.

But then IRL turning is possible with one track / side of wheels operational, not both. Even if it will be slower and turn radius larger (depends on a vehicle).