Check out this shift?

In the late nineties and in the year of 2000, you had a handful of drivers that you expected to win races and then a couple of dark horses. Those drivers were Jeff Gordon, Jeff Burton, Dale Jarrett, Bobby Labonte, Dale Earnhardt, Mark Martin, Terry Labonte, and Rusty Wallace (Jeremy Mayfield and Tony Stewart coming later). Darkhorses included the likes of Bobby Hamilton, Johnny Benson, and John Andretti. Then all of a sudden, from 2001 to 2003, you had a wide range of different winners. I think the average between the three years was sixteen or seventeen. I know that in 2001, 19 different drivers won races. In 2004 there was a bit of a transition, although I recount one darkhorse winner in Joe Nemecheck. But since 2005 there again has only been a handful of drivers that win each year, and the only constants between 2008 and now have been Jimmie Johnson, Denny Hamlin, and Kyle Busch. What constitutes for this shift?