I never really understood why he was that famous, maybe because of the sect concept, letting young girls do his bidding, or maybe because they killed a famous person. Maybe it was because of the time it happened and/or because of how evil he looked/wase compare to others.
When you take a look at the Green River Killer Gary Ridgeway, the dude killed over 100 women, some they couldn't even found the remain or identify them as even him couldn't remember all that he killed. He was 10 times more deadly than Manson ever was, yet couple years ago i didn't even knew who he was before stumbling across a very poorly made cheap movie about him. Yet i know the name Charles Manson since i was like 10 years old... (Granted Marilyn helped popularise the name too)
But when you look at Gary he look like that typical middle class man, he does not look evil or even sound evil (from the video i seen of him) , but Manson seem to personify the devil incarnate... I supose its probably that...