They should have retired in 2008.
Politicians have a tendency to stick around during what look like good cycles, and to leave before bad cycles. It's a bit like the snake eating its own tail. The fear of being in the minority leads to popular incumbents quitting which increases the chances they'll be in the minority.
That said, some incumbents are in trouble as well. If Rockefeller had left in '08, there's no indication that the Democratic candidate would be as strong.
Andrew Gelman posted about this on
538.
