Heavy tanks are generalist units with no major weaknesses that are capable of combating everything on the field hence why the tanks are limited to 1. Even a long range tank destroyer will struggle to deal with a heavy alone. Before the limit was imposed the safest strategy was often to just try to produce more heavies than your opponent. This is not the case for arty which has a very clear weaknesses to offmap strikes (for static arty at least) and zero field presence.
Speaking of which, that's something I'd gladly have changed at the cost of being limited to one of them.
Stuka Dive Bombs and other like ensuring any static howitzer is a waste of time and MP just ain't right.