Slitherine

All these early light artillery pieces (Sturmpanzer I, II, 38(t)) were not very good, not reliable and not well armored enough to get really close (so having them be very vulnerable while still short range is exactly how they were). That's why the StuGs were developed. They used the short barreled tank gun the PzIV had had initially used for infantry support. That 7.5 cm gun was perfectly adequate for the role and the vehicle was well armored enough to get into position while being well protected. So all these other tanks were basically failed StuG prototypes. The StuG was really the deal, everything else was just patchwork because nobody had really though about how to build a close range infantry support artillery vehicle, so they were just taking a tank chassis that was cheap to build and put a gun on top. Heck on the Sturmpanzer I they even let the wheels on the thing so you could actually dismount the towed gun from the chassis if necessary. It was the uncahnged gun with wheels and all just stuck into the compartment of that thing.