The allies didn’t “allow” - let’s be blunt, there was little they could do. The Italian campaign had hit a stalemate and it took until 1944 for D-day, while the Russians were already hammering in Poland.
There was no way to prevent the Russians getting to Berlin first.
But what if the US had not supplied the USSR with so much material that helped enabled it (despite Stalin's absurd programs executing military leadership) to become an advancing army, getting to Berlin first? What if humanitarian assistance was given but only enough military aid was supplied in order for Stalin to stave off Germany? Lots of "ifs" in history as well as Scripture.
War could have ended months earlier had Patton been given fuel to continue pushing once the breakout occurred in France as Germany was on the ropes. Instead they diverted resources for market-garden.
Decision to let Soviets take Berlin was made in Yalta.