Short version: because traditional characters, while they make more sense while learning them, are a pain in the ass to write and look very intimidating. The stated benefit was that more people would become literate, which happened (for various reasons)

Longer version deals with Mao's apparent desire to replicate the First Emperor, revolutionary doctrine saying out with the old, and just general "uplifting the workers" themes in Communism. The benefits of switching to simplified can be seen by comparing the character 认, "to know" with its traditional counterpart, which I can't write on mobile.