Why did Mao become a Marxist?

I don't know

Mao envisioned that he could bring revolution to a rural country where the peasants could be the true revolutionaries. By joining the communists the peasants also gained access to more land (as described in your answer).

Holding that "political power grows out of the barrel of a gun", Maoism emphasizes the "revolutionary struggle of the vast majority of people against the exploiting classes and their state structures", which Mao termed a "people's war