The mortar was necessary for anti-garrison. Knowing your opponent was going to rush a building and not being able to do anything about it was terrible.
Welcome to the Axis life.
So please tell me what the OKW player starting north on Semoisky is going to do against a UKF/Soviet player who rushes an MG into the capping point building at the base exit?
The whole city is closed for you then until the mid game and the enemy can defend the other fuel point with his whole army (except the MG inside the house).
OKW has even less anti building tools in the early game. Flame nades come with a minimum of 35 fuel (Medic Truck) or 60 Fuel (Mechanized Truck). Meanwhile there is absolutely nothing OKW can do against an MG inside a house especially on said building on Semoisky.
USF can bite the bullet and research nades early to get smoke to close in and nades to clear buildings. OKW either needs a Stuka (100 Fuel!) or a Medic Truck (pretty non-meta right now) with LeIG what leaves you vulnerable to light vehicles because of the lack of a Puma.
If USF got a mortar, OKW could at least unlock the LeIG in T0 like the MG34 after any truck was built. OKW needs it more than USF did.