His popularity translates to money. Somehow, his clown suits and blow up dolls appealed to millions of people that fell fanatically in love with him. Carmello seized on that popularity to manipulate the sport in his favor to make himself a very rich man. Still, he had to go out and do it and was good enough to do just that against much much weaker competition. When you mix together a very good rider, weaker than the norm competition and manipulation of the sport towards that rider, its not hard to see how he became a 7 time GP champion. With that being said, his advantages paled in comparison to what Ago enjoyed throughout the majority of his career when he was on machines that were 25-30 mph faster than anything on the track. Rossi's record is impressive for someone who doesnt know or doesnt care how he got it, but my belief is Stoner, Lorenzo, and Marquez are superior talents who will never get the credit they are due because of the narrative Dorna planted and nourished for 15 years. False idol comes to mind when looking for words to describe Valentino Rossi.
I also think you are missing a component of his appeal. You may not agree, but to me Rossi is more of an average guy than a typical motor sports athlete. He's not a GQ looking model-type athlete like some of the F1 boys, for example. Rossi is in essence a goofball, who remains close with his childhood friends, something very few top level athletes do. Again, I think he appears more like a "regular guy" than a stud athlete. I think this appeals to many people today, in a world filled with vanity filled tabloid publications.