1. Because of the ease of travel and the internet, there are more people watching sports than ever before.
2. Klitschko is a rare white boxing and draws more white crowds. This doesn't mean he's the best boxer ever.
3. The only reason there are fewer American boxing champions in the modern era is because boxing's popularity has declined and the athletic youth don't care about boxing.
4. Like @
Rev says, more spectators doesn't mean more money. When you want to increase the demand of a product, you lower its price.