Stats. If you look at his regular season stats, his PPG is light-years ahead of Rondo (13.7 v. 20.8 ). That may be an unfair comparison considering Rondo is a better passer/rebounder/defender and really turns it up when it matters most in the playoffs. But averaging 20.8 for a PG in today's league is really amazing.
Also, while I think Rondo is a helluva PG, I don't really see him as a threat to take over a game scoring-wise. He dominates the game in other ways. Rose is the type of player that could completely go off for 40+ points, and no one would blink an eye. He has the scoring mentality of a 2-guard or SF.
So, whether fair or not, public perception will always favor the bigger scoring threat. I'm not a Celtics fan, so maybe I am undervaluing Rondo - but if I had a choice between Rose and Rondo for the next ten years, I would likely choose Rose (because of his ability to carry a team with scoring, and his ridiculous ceiling).
Overall though, we are really in the golden age of PGs. The 90's and early 2000's were horrible. Think about it, Marbury/Francis/Davis were considered the pinnacle of PG earlier this decade. Kidd/Nash are the only two truly great PG's that played earlier this decade. But now we have Rondo, Rose, CP3, Williams, Nash, Kidd, Tyreke Evans, Brandon Jennings, Stephen Curry, Nelson, and soon-to-be Paul Wall. Special talent right there.