luv2run can speak for himself, but in Lance's case his choices seemed to be going pro in either cycling or triathlon and the money was better in cycling. He was supposedly a good enough swimmer that he might have been scholarship material. Running is his weakest discipline so pursuing running wasn't much of an option.
Compare that with Lukas Verzbicas whose triathlon strength is clearly his run, followed by bike, with swimming as his weakness. Given that skill set it makes sense to pursue running as a career which seems to be where he's headed.
Finally, take someone like Jerrod Shoemaker who is a pretty competent runner, but not quite good enough to make much of a living at it so he's an ITU guy.