Why did the Soviet Union join the Allies?
The Soviet Union joined the Allies because Nazi Germany invaded the Soviet Union.
Nazi Germany and the Soviet Union had a non aggression pact. This allowed Germany and the Soviet Union to invade and divide up Poland. It also allowed the Soviet Union to annex the Baltic States.
Germany then was able to focus its armed forces on France and United Kingdom. In a lightening attack Germany destroyed the France army and conquered France. The air battle to pave the way for an invasion of England failed.
The Soviet Union stood by and did nothing to help France or England, during this time.
When Germany's attempt to conquer England failed Hitler turned his attention to the Soviet Union. When Germany broke the treaty with the Soviet Union the Soviet Union asked to join the Allies in the fight against the Axis Powers.
The United Kingdom, and America were already fighting the Axis powers of Germany, Italy and Japan. The enemy of my enemy is my friend. Since the Soviet Union had a common enemy of Germany with the UK and the US they became friends.
America and England shipped tons of war material to the Soviet Union through the Murmask Convoys. This helped the Soviet Union defeat the Germans in Stalingrad. The opening of a Western Front in North Africa, Italy, Southern France and finally Normandy helped to relieve the pressure on the Soviet Union.
By becoming Allies with the (despised) western powers the Soviet Union was better able to resist the invasion by the Nazi war machine. Still the Soviet Union took the brunt of the fighting of the war suffering the most dead and wounded of any nation in World War II.