Since Paul has been in the league, he's been the often ignored star player in Boston, however with Rondo he has catapulted to superstar level in the league in terms of marketability. I've seen more commercials with Rondo than I have with Pierce. He's selling more jerseys and whatnot.
What I don't understand is how Rondo became so popular despite not being quite as good as Pierce has been for the celtics, even in the early goings?
What makes Rondo so marketable?
I think you might be overstating the difference. I'll focus on jersey sales.
This year, for example, Rondo was 8th but Paul was a quite respectable 15th.
http://www.usatoday.com/story/sports/nba/2013/04/09/top-selling-nba-jerseys-team-merchanside/2066321/In previous years the difference has been bigger or smaller - Rondo was 3rd in 2010-2011, for example. But if you go back to the 2008-2010 period KG and Pierce were in the top 10, with KG often being top 3 and Pierce being close to top 5, with Rondo much lower.
So, I think you could say that measured this one way, their popularity has tracked performance pretty closely.
It is odd, though, that Rondo has been higher than players like Durant.
And I just looked through several lists going back to the mid-2000s, and Timmy Duncan is absolutely nowhere to be found on any list, while guys like Nate Robinson will sneak into the top 10 from time to time.
So, the puzzle you talk about does exist - I wouldn't say that Rondo vs. Pierce is the biggest, though.