Why weren't British or American troops ever sent to the Eastern Front to support the Red Army?

Ground troops were never sent to the East.

Many senior Soviets, including Stalin, were wary of the Western Allies. They were old enough to remember the last time British and American troops had landed in Russia: it had been 1918 and they'd been trying to suppress the Bolshevik revolution! Stalin certainly didn't want Western troops he couldn't completely control fighting in his backyard.

However, there was an effort to temporarily base American bombers on Soviet soil for "shuttle bombing" raids. American bombers would leave bases in Western Europe, bomb Axis targets and then land in Soviet-held territory. Then they could reverse the routine and return home, bombing another target on the way back.

The impetus foe this came entirely from the Americans, not the Soviets. The official USAAF history explains:

Since the early days of the war AAF leaders had been attracted by the idea that shuttle bombing between widely separated bases might pay huge dividends. The experience gained with shuttle operations between British and North African bases, notably in the Regensburg-Schweinfurt mission of 17 August 1943,had introduced a note of caution into AAF planning, but opportunities for shuttle bombing were among the reasons advanced for the establishment of the Fifteenth Air Force in the fall of 1943and by that time the United States was urging upon the U.S.S.R. the use of Russian bases for the same purpose. It was well known that the Germans were relocating many of their plants in the east, and it seemed reasonable to expect that American bombers operating out of Russia could strike targets in eastern Germany which were beyond the reach of aircraft flying from England or Italy.

It would be helpful, moreover, to compel the Axis to spread out its defenses against air attack and to impress upon the enemy high command at about the time of OVERLORD that Germany was exposed from all directions. Perhaps of even more importance was the desire to demonstrate to the Russians how eager the Americans were to wage war on the German enemy in every possible way and to gain from the Russians a fuller appreciation of the contribution of the strategic air forces to the war effort, for to date they had revealed scant regard for the work of the heavy bombers. And if these shuttle operations proved effective, it might be easier to secure Soviet approval for the use of Siberian bases later on in the war against Japan -- a consideration seldom lost sight of by the American high command. The manifold advantages expected from FRANTIC, as the project came to be called, were considered ample justification for the effort and expense involved.

However, the experiment was soon nixed after a few missions in 1944. Soviet leaders had always been reluctant to accommodate the American bombers, for the a variety of reasons (anti-Western sentiment, suspicion of outsiders, bureaucratic turf wars, etc). Soviet commanders wanted the bombers to bomb different targets than the ones the USAAF wanted to hit. The logistical hassle of staging ground crews, bombs, and gas so far away from the bombers' home base further complicated things. That, and the Luftwaffe caught the bombers on the ground at Poltava and destroyed dozens of them by launching a massive night bomber raid to plaster the airfield.