TDF is a much more focused target audience than the other sports you mention. Average Joe American isn't going to be casually tuning in to watch a sprinter's stage at 8:30am EST. Cycling fans and enthusiasts will. So you're going to get pretty much nothing but cycling related commercials or endorsements by cyclists.

Let's make you a marketing director. People watching on VS are Americans. So you're going to want an American endorser. And nobody outside of the sport's enthusiasts know who Levi Leipheimer or Tyler Farrar are, George Hincapie may be slightly less obscure. But Lance? Everybody knows him even if they've never seen a cycling event or Sports Center in their life. Maybe Lance is a little more expensive to get, but now your commercial can be used for more applications outside of running during the TDF (mmmm, Michelob Ultra) so you can spread out the product costs.

It's more Lance overkill than usual because he's on an American team with a consumer product retailer corporate sponsor. So they're going to ***** him out as much as possible.