Dane Beardesely was the Last legitamately pro level rider to come out of the U.S and he's in his early 30's...Thats frightening...
Justin Miller came out after Dane. He hasn't ridden since the first Jomopro though.
Wilhelm said at Dew Tour he isn't really interested in competing any more. He can just do shows and make more money. I know Cerra does a bunch of shows every week and the payout is consistently better than contests.
There is really no incentive for people to go pro in the US. Even if you do, sponsorship is extremely limited to a very select group of people, and if you don't have the right connections there's very little that will come your way. Travel costs have gone up in the last few years, and payouts at contests (compared to the Xgames days) have dropped significantly. The only reason guys like Ucchie, Matthias, Viki and other riders like that come to events like Dew Tour from Europe and Japan is that someone is covering all their travel costs. Even if you get a whole bike for free, you'll still be paying 500-600 just to get to a contest that more than likely you will not win unless you've brought tricks that can beat the guys who ride full time for their sponsors that pay everything. It's a huge up hill battle
On the other hand, you could "go pro" just for the sake of doing so and not get anything in return aside from another contest experience. Simply throwing more pros into the mix doesn't mean much if they can't place well because there's such a small return on investment.
Short answer: there are no new competing pros because there is almost no reason to do it unless you want to live on very little.