This is one of those groups that has that "Pixies" effect. Where no one really knows about them when they were officially a band - and then YEARS later, they become popular.
MC5 was kinda early punk, fast hard rock. They came from Detroit in the early 1970s.
Now that Detroit is reinventing itself, becoming trendy, and all kinds of life is starting emerge from it - people around the world are FINALLY realizing that Detroit is like a fertile field for emerging music and acts. From soul, Motown, funk, rap, iggy pop, techno - Detroit is a true originator of great music.
So people found out about MC5 probably because of this realization of Detroit music prowess.