Can anyone give a brief yet clear explanation of why the Soviet Union was rising so fast after WW2?

Because Europe was turned to rubble from WW2, and they needed extensive repairs, and communism is a very fast way to deal with problems, though it almost always ends up backfiring in the long-run. So many countries that were having a hard time repairing welcomed communism with open arms, because it would help fix their country. Eventually the Soviet Union got very powerful, and America was against Communism, so they started to race, and the communists tried to turn many other countries into communists, even if they didn't want to.