I think it's a combination of things, rather than one reason in particular:
1. This was Manu's breakout season and before now very few casual basketball fans had heard of him outside of Texas. He didn't come up through the college ranks and get lots of exposure from a young age that way like Duncan did, for example.
2. He plays for a small-market team that doesn't have big media types to hype him up. (i.e. not NY, LA, Chicago, Miami)
3. He's modest and doesn't hype himself by making controversial statements, getting into messy contract negotiations, or engaging in criminal activity.
4. He cares more about team play and winning, rather than personal numbers, so he's not trying to post 30-15 numbers on a regular basis and go for the monster dunks just in order to make the nightly highlight reels.
Call me naive, but I believe his ethnicity and national origin have relatively little to do with his lack of exposure. Americans love winners, and if Ginobili continues to shine like he did this past year, they will get on the bandwagon. Especially if he's marketed well through endorsements. The Latino population is one of the fastest-growing demographics in the country and has an increasing amount of disposable income to spend on shoes, hamburgers and cars that Manu sells. Advertisers are crazy if they don't take advantage of the opportunity he represents for them. With time and more exposure, Manu will get more acknowledgment from the general population.