Some of it was a question of seniority and how many jobs were available, and finally the timing of decisions.
For Barbarossa there were a total of 4 Panzergroups, army-sized formations made up of motorized and armored corps. 1 in AGSouth, 2 and 3 in Center, and 4 in North. Each was commanded by a Col. General, the equivalent of a Western 4 Star. And each Crops was under a Heer full General, equivalent to a US/UK 3 Star LTGEN. Rommel had been promoted to Heer Major General(equivalent to a Brigadier in the US/UK) in Summer 1939.
Not out of sorts for a division command, but that and Heer LTGen above it was not in the normal order of things senior enough for Corps commands. In fact, unlike against France the year before, by the time the German attack on Russia began there were no Generalmajor’s in command of any of the armored or motorized units after rounds of promotions and shuffling everything was far more uniform.
So why not just promote Major General Rommel again? After all, he would become LTGen Rommel when given his scratch command of 1.5 divisions in Africa in early 1941. He’ll make it a double promotion to full General of Panzer Troops and give him one of the 11 mobile corps in the Panzer Groups.
But then who do you swap out? Maybe someone like von Mackensen who was from a more Staff officer background in the cavalry but who could be relied upon more to keep the insane logistics running? Certainly not someone like von Manstein whose planning chops had won him enough backing to get a battlefield command following his drafting the plans to send the Panzers through the Ardennes. Certainly not say Geyr von Schweppenburg who had already proved he could handle an armored corps by leading one in France.
There is also the intricate personal politics of the officer corps. Along with the timing of the decision to offer units to support the Italians in Africa.
Plans to send a modest force there have their origins as early as summer 1940, and in September MGEN von Thoma, commander of 3rd Pz Div, was in Africa to take a look around and offer ideas on getting a division over there. He was no stranger to expeditionary warfare having fought in Spain and suggested that a larger force, closer to 4 divisions than the 1 being considered was really needed. However this was not acted on right away, shoring up Italians positions against Greece took priority, and the fighting seemed in hand in Africa. It was not until February 1941 when British attacks destroyed the Italian front that things got to happen.
This formation, built around 3rd Panzer (re-designated the 5th Light Div), was to be under MGEN von Funck, an officer who had served in staff and Panzer rolls before. But in the 3 weeks between January 15th and February 5th something changes. At that point, OKH Chief of Staff Halder writes in his diary that Rommel is to be the new commander of the force. Why this happened is not easy to pin down directly.
Certainly, Rommel had direct ties to Hitler from previous positing during the Polish campaign. and von Funck may have been on Hitler’s blacklist as he had been an aid to von Fritsch with whom Hitler had clashed before the war. But the matter was settled then, con Funck took Rommel’s old 7th Panzer division into Russia and Rommel goes to Africa.
A few weeks later and Rommel might have decided to not press or accept the command while planning for Barbarossa had started in the summer of 1940, wide-scale briefings of senior leadership would only really get going in the first half of March 1941.
We wouldn’t say it’s a matter of a 2nd rate general to a 2d rate posting. Certainly, Rommel had a reputation for not being easy to command or staff for. But he also was given some accomplished officers for he key subordinates. von Thoma would go to Russia then come back to Africa, Prittwitz was in the same vein as Rommel and would be killed at Tobruk leading from the front, or von Ravenstein who took over 21st Panzer after leading motorized infantry as part of a panzer div from Poland through to the Balkans in spring 1941.
In fact, that flow of newly promoted officers from Russia and elsewhere to Africa shows that it was as much a timing and politics thing as it was a judgment on capabilities of a still relatively junior panzer general.