It depends on the time period in question:

First, the original assault guns were not anti-tank weapons. They were anti-infantry/stronghold, hence the word "assault". The original stug 3 was a short barreled weapon.

The reason not to have a traversing turret is that the traversing turret was expensive and also has a higher profile (i.e. easier to target with anti-tank weapons).

As experience with the eastern front went on it was decided to put anti-tank weapons on these because the Germans needed cheap and effective anti-tank weapons. The lack of a traversing turret turned out not to be such a bad thing.

For example: A tank is always a trade-off between speed/mpg and armor. So, tanks have very heavy front armor with less side armor and even less rear armor.

The experience of the 75mm stug III's turned out well, and when their commanders transferred to tanks with traversable turrets the result was amazing. For example, the legendary Michael Wittman started out as a Stug 3 driver. He did so well as a tank commander later because he never traversed the turret, therefore his Tiger was always displaying it's front to the enemy.

The German's picked up on this and decided to make the Stug 4 as the war winning tank but were never quite able to build enough of them.

Edited to add: It is all well described in Guderian's book: http://www.amazon.com/Panzer-Leader-Heinz-Guderian/dp/0306811014/ref=la_B001ITYGCU_1_1?s=books&ie=UTF8&qid=1410828585&sr=1-1

EDITED TO ADD AGAIN: So, when the German's "kicked in the door" in the east they were surprised to find the KV1 and T-34 tanks, and their tanks and anti-tank guns couldn't do much about it. They finally developed the 75mm gun. However the standard tank at the time (the Mark 3) could not fit it into its turret. The last of the mark 3's had a 60mm gun. Anyway, they could, however, fit the 75mm gun into the Stug 3, so that is where it all started.