So imagine Wehrmacht having 5-10 different artillery diameters. In the time of war - that is constant shortages and supply delays, how do you supply your artillery forces - when all guns have different sizes?
That's because Germans weren't very bright or prepared for war.
Btw, in WWII, the main factor wasn't even tanks or planes, it was artillery. Russians, by far, had the best artillery. No one was even close.