Couple of reasons jump to my mind.
Mammoth is a pretty long way from a decent-sized airport, making travel to/from Boulder a longer day that I might face coming from NJ (which is about 5 times as far away). And I don't know if you've trained for a marathon, but p---ing away the 4 or so days to travel and race makes a big difference the Flora London Marathon 9 weeks away.
Plus, if he were to make the team, maybe he is not interested in even-longer travel to-and-from Kenya -- right during the peak marathon trainign weeks -- only about a month before ANOTHER long trip to London.
And if that's the case, for a guy whose livelihood depends in part on being a likable promotional vehicle, making a US team (potentially winning the nat'l championship) and then declining the invitation to represent the USA in international competituion may not look too palatable and might run him into a "bad guy" rather than a "good guy" real quick. "Bad guys" don't get good endorsement or appearance pay. Hey, the guy's gotta feed his family.