Training wasn't going well enough to predict an American best, and the 15k confirmed that, so no point to send him so far away when there's work to be done. That's my guess.
He did run too many hard marathons in too brief a time and he probably erred in thinking that because marathon was his best distance, he should just train for and race marathons, instead of working seriously on his 10k speed. Noakes goes over many great runners who ran their best marathon within their first three or four, and who declined considerably if they ran too many too soon, because of permanent muscle damage. But he'll have raced, I believe, just once in about eight months when he gets to Boston, so the recovery should be adequate for this one. After Boston, I say that he should delay his next marathon until fall 2010.