Dave, I think you're looking at the wrong guys. The fighters that "make it" may not be all defensive-minded, but they are out there.
The Irishman who just fought Amir Khan was definitely what I woudl call a defensive minded fighter- he was extremely hard to hit and moved very well. Calzaghe was another, and Martinez is another who could be called fast and slick, although he may not be considered defensive (OK, not at all, but you know what I mean.) Dickie Eklund, Micky Ward's brother, was another example of a good slick fighter.
Keep in mind that very defensive minded fighters are hard to come by in the first place- no matter what race they are.
There's a couple factors that play into it, one of them being the socio-economic ones at play before a white guy even enters the boxing gym. Keep in mind that I'm a democrat when I say all this- but I do believe that a white kid boxing is going to have a higher likelihood of having grown up with a more stable family, he probably lives in the suburbs, he's probably going to college, etc. If he's boxing, he's not considering it his career- it's just something to do.
An old trainer from Jersey City told me that once long ago, that none of the Italian kids want to fight anymore, "They all want to go to college!" he said. He was very exasperated by it.
So the proportion of white guys boxing is much smaller, and the ones who are going to take it seriously is even tinier now. So it's not that you couldn't have slick white defensive fighters- Pep shows that they could be some of the best. But you just don't have the high number of white guys boxing in this country.
Now, when you do get white guys fighting, they come generally from Europe, and 99 percent of European trainers are, frankly, garbage. They teach their fighters to come in on straight lines, they throw 1-2s and never vary, they don't move their heads, etc. This is why I'm so America-centric in boxing- our trainers are much, much better, and they make our fighters much, much better.
I'm not sure why this is- maybe its the heavy African-American influence on the sport here- but i can tell just in the way that I've been trained by different guys - the black trainers will want you to work your footwork more and more often, they want you to dance, they want you to not get hit, and they really are teaching you a different game than the white trainers. Personally, I love the way they teach, and I've never wanted to move like a white fighter.... one of the biggest compliments I've ever received in the gym was a pro fighter looking at me and saying "He moves pretty good for a white guy."
Third, I think that a lot of white guys get into a self-fulfilling prophecy of wanting to be the next Aturo Gatti or Micky Ward. They want to bang, want to be like their heroes, and so they get caught in the trap of standing toe to toe. You'll never hear some New Jersey guinzo say, "Man, I want to be JUST like Winky Wright."
These guys don't realize that it sucks to fight like Arturo did, and it fucking hurts.