He was one of the biggest names during the Attitude Era so a lot of fans who watched during that time period have fond memories of him and are marking out that he's back again. He's also one of the few who is allowed to swear during RAW and I'm pretty sure he gets a lot of freedom when it comes to his promos (which is kind of unfair to the rest, yes, but he has earned it), and even though the product is PG most fans like the attitude stuff anyway. He also hasn't been exposed as much as someone like Cena who's been on our TV screen non stop since 2005 (longer than Rock's full time run) and pushed down our throats.
By the way he did get booed in 2003 remember which is why he turned heel to Hollywood Rock. Which then made fans like him again