11 slams > 6 slams, simple as that. Most popular peoples in sport are always ones which are more succesfull.
If popularity would be based on charisma and character, Federer and Nadal wouldn't be in the top 100.
Ah what?? Not denying that to be really popular you have to be successful. That is true, but your last sentence is just plain flat out wrong. If you don't think Federer and Nadal are in the top 100 based on charisma and character you are deluding yourself. In fact, they are 1 and 2, which is precisely why they're so popular.
In regards to why Nadal is more popular than Djokovic, well about 5 or 6 people have already said it. He was Federer's chief rival first, plus he has earned the title of King of Clay, which helps his marketability totally separate from Federer.
I don't think it really matters how many slams Djokovic gets. He'll never be as popular as Nadal frankly. Of course he has the whole early career image that he has pretty much repaired now (re: retiring and the like), but honestly, as emotional as Djokovic can be on a tennis court, he's more like Federer than he is Nadal.
Nadal's running to the baseline, and general exuding of energy on a tennis court is totally different from Novak's. Quite often Novak will just roar up to his box after a big point, whereas Nadal will do double fist pumps with pelvic thrusts while sprinting to his chair, or falling on the court (i.e AO 12). He actually did this a lot when he was younger, and I don't mean it as a bad thing. I think this is one of the reasons people have liked him through the years. Other people like Novak for different reasons, but if you don't like his dancing to Gangnam Style or Carly Rae Jepsen, he can be a little bland, impersonations, which he barely does these days not withstanding. No offence to any Djokovic fans.
And while I'm at it I might as well say what I think about Federer. I can see why some would call him boring as it pertains to showing emotions on a tennis court, but he's just different from Djokovic and Nadal. I wouldn't say he's immune to showing emotion. Sampras wasn't immune to it either, but I think it just shows that if you don't go a bit over the top (for lack of a better term), you're classified as boring. For Sampras, he was "boring" because he was compared to Agassi, and for Federer it's the same case with Nadal and Novak. Unlike Sampras however, I can see Federer's personality. He can be very funny in interviews, especially with Courier. I never got this impression with Sampras.
In regards to his emotions on court, he's had plenty of moments where he's been really fired up. Everybody remembers his finger wag at RG 11, (which some thought was over the top, which I found incredibly funny, but I digress). Or how about when he tied the 08 Wimbledon final at 2-2 sets? Or when he broke Nadal to go ahead in the 5th the year before? He's had more than that too. Point is, he's had a lot of emotional reactions over the years. He's not a total robot, but you have to pay attention or you'll miss it, whereas this is not the case with Nadal or Djokovic.