I'll take the claim at face value, provide some historical and mechanical background, and make an educated guess.

It would be because after the Fall of France the Western Front consisted primarily of attacking with or defending high flying bombers, while the Eastern Front was focused on ground pounding.

The Supercharger

First, a word about a critical and expensive piece of equipment: the supercharger. This was the key difference in the high altitude performance of aircraft. Combustion engines' primary fuel is air. This mixes with the petrol to combust. How much fuel you can burn is limited by how much air you can pull into the engine. At low altitudes you basically get as much air as you need for free. But at high altitudes the air pressure drops, the amount of fuel you can burn drops, and performance drops.

In WWII they compensated for this with a supercharger. It's basically a fancy name for a fancy air compressor. As the aircraft climbs and air pressure drops the supercharger compensates by using some engine power to shove more air into the engine to keep the air pressure optimal. At high altitudes the trade off between using power to run the supercharger is worth it. This is in contrast to a turbocharger which uses engine exhaust to run its compressor and takes away no engine power.

There are various technical improvements to a supercharger to allow high performance across a wider range of high altitudes. The ultimate for WWII was the "two speed, two stage" supercharger. These were complex devices requiring careful management by the pilot to avoid blowing out the engine.

Aircraft Engines

The availability of engines is one of the biggest limiting factors in wartime aircraft production. Aircraft engines need to be lightweight, fuel-efficient, rugged, high performance at a range of altitudes, and continue to operate with various crazy forces applied to it like flying upside down. They're expensive to design and produce, and finicky to maintain. Often good aircraft designs were hampered because the intended engines simply were not available.

Any feature you didn't need on an engine was just extra weight and cost and maintenance. For aircraft intended to fight at low altitude a supercharger was just that. You don't need a supercharger to conduct air-to-mud operations. Instead they often used a turbocharger, more efficient at the intended low altitudes.

The Rolls-Royce Merlin Engine

The most famous example of the impact of a supercharger was in the development of the Rolls-Royce Merlin Engine which powered the famous Spitfire and, later, the P-51 Mustang. During the Battle of Britain the Spitfire's performance at high altitude was not so great. Continuous engine improvements saw it become a superb high altitude fighter. Its high altitude performance continued to increase until it had a significant advantage over Axis fighters at the high altitudes required to protect the heavy Allied bombers. The Axis struggled to catch up.

In particular the success of the P-51 Mustang as a high altitude fighter was in large part because of its engine. The P-51 was originally introduced with an Allison turbocharged engine which underperformed over 15,000 feet. The British and Soviets were not impressed. In 1942 both the British and Americans experimented with putting the improved, supercharged Merlin into the P-51, and the legend was born.

The Western Allies Bombing Campaign

In the West between the Fall of France and the Invasion of France the Western Allies' are engaged in continuous strategic air battles. The Battle of Britain in late summer 1940 and subsequent Blitz into mid 1941; the Battle of the Atlantic involved many high flying and long range attack and patrol aircraft through the whole war; and the Defense of the Reich required high flying bombers and high performance fighters to attack and defend them.

The obvious reason is because all the Western Allies had left was Britain. That island precluded conventional land warfare, so strategic air attack it is.

The not so obvious reason is because Britain and the US had invested heavily, both in resources and psychologically, in the idea that strategic bombing could win the war. They invested heavily in heavily armed four-engine bombers able to carry a large bomb load into the heard of Germany and were willing to accept the high losses of continuing the campaign. But because of the belief that "the bomber will always get through" and that the bombers would be able to defend themselves, it would take some time before they had a high performance long range high altitude fighter to escort them.

Of course the British were fighting in North Africa the whole time, and after Nov 1942 so were the Americans, and everyone went on to Italy. Those theaters required low-level tactical air support. But those were side shows compared to the aircraft being chewed up in the strategic air campaigns. You don't need a supercharger for air-to-mud operations.

The Eastern Front

In the East the strategic situation was rather different. The largest land war in history was raging and that's going to require a lot of low level tactical air support, and the fighters to attack and defend those strikes.

Germany

In the West, the Germans had to respond to the increasing number of pesky high flying heavy bombers, and their increasingly effective escorts. This required increasing numbers of high performance, high altitude interceptors for defense.

The Eastern front was an entirely different matter.

In contrast to the US and Britain, Germany did not seriously invest in strategic bombing. The Luftwaffe was built around supporting the army. They thought the war would be over quickly. Since the German plan was to conquer only up to the Urals, they did see a need for a "Ural bomber" conceived to strike at Soviet industry beyond the Ural mountains. But in typical Nazi German fashion it was cancelled once the person pushing for it died. Germany continued to vacillated throughout the war about strategic bombing starting and cancelling and restarting and altering their strategic bomber programs until it was far too late. Germany only managed to get one heavy bomber comparable to the Allies off the ground, the He 177, but this was plagued with engine problems in part due to the requirement that it also be capable of dive bombing showing how little confidence the Luftwaffe had in strategic bombing. Instead they increasingly put their faith and resources into wonder weapons.

Lacking strategic bombers in quantity meant they had to stretch to the limit just to strike Britain and out into the Atlantic. They had no hope of attacking the US, nor Soviet industry which had moved deep behind the lines.

Soviets

The Soviets did invest some in long range bombers, but only had one true heavy bomber, the TB-7/PE-8, and only in small numbers. They found much of their air force destroyed in early attacks. The rest, even aircraft intended for strategic bombing, were pressed into tactical strikes to directly shore up the always in crisis front line. As the line pushed further east, strategic Axis targets were further and further out of range.

This is not to say there was no Soviet strategic bombing. Early in the war they hit the Romanian Oil Fields, naval aviation attacked Germany, and the newly formed Long Range Air Force (ADD) conducted strikes deep into Axis territory. Once on the offensive, Soviet long range strikes first concentrated on disrupting Axis rail and transport, and then strategic industrial targets. But this was less than 1% of Allied strategic bombing sorties.

Further References

• Development of the Rolls-Royce Merlin from 1939 to 1945
• B-17G Standard Aircraft Characteristics