Because Nazi Germany had many different organizations, some of which were not under party hierarchy. The SS for instance was not a Nazi party organization a Nazi paramilitary organization distinct from the rest of the party and thus had their own uniforms, and in the SS there was the combat branch (Waffen-SS) which had their own field combat uniforms. Then there's the Gestapo which was officially part of the police (RHSA) rather than SS and wore variations of the SS field unform in occupied territories. Each branch of the German military had their own uniforms of course, and some specializations (i.e Panzer crewmen) would have uniforms different from others.
So without going into too much detail about Nazi organizations, it's kind of like how the US government personal have different uniforms from army to Navy to Air Force to NYPD etc.
Consider that German ground forces were not as homogenous as their opponents. They had several organizations involved in land warfare: *The Wehrmacht (German Army) *Fallshirmjager (Paratroops, which were under Luftwaffe administration) *Luftwaffe Field Divisions (Redundant Luftwaffe personnel organized for land combat late in the war) *Waffen SS