Manu is a guard...Guard is the glamour position in the NBA..C/PF isn't. Great Guards are like Lambourghinis...whereas the bigmen are like Semi Trucks...you never hear anyone oohing or ahhing over a semi-truck, no matter how magnicifient it is...but you do hear people ooohing and ahhing over fast cars. No one ever wants to own a big semi...but they love those cars.
It's like that. The shoes of bigmen never sell...not Hakeem, not Shaq, KG, Duncan...it's always the guards that sell the most shoes and merchandise.
I'd also say that due to the style of play he attracts a vocal and energetic type of fan...
I do not think that necessarily means he is more popular or beloved than Duncan or (especially) Robinson. He's just more cheerable.
As for the worldwide thing...that's a different dynamic entirely...a lot of international basketball fans love seeing a guy that built his career in their basketball world hitting it big in the NBA.
Manu is probably my favorite Spur to cheer and watch. I'd probably buy his shoes before Duncan's.. and he definitely has a winning impact on the court...a huge one. But that doesn't mean I'd take him first if I was building a team. I think it's similar for a lot of Spurs fans.
AJ and Steve Kerr used to get incredibly loud cheers...well no one is taking Kerr over Duncan...and there are only 2 Spurs fans I know of that are mentally impaired enough that they would actually take AJ over Duncan or Robinson(they know who they are).