I may be wrong and misreading translations but I think while Kawauchi was initially in the elite field there may be a rule in London that you can't race a marathon (maybe all-out race) within a certain amount of time. Osaka in Feb. may have been too close. I believe Mongolian NR holder Ser-Od Bat-Ochir was supposed to race London in 2010 but was taken out because of his recent marathon exploits - he then won the inaugural Brighton Marathon.
Basing much of this off of this:
Translated section on Google Translate he says "Withdrawal from the elite due to misunderstandings in communication due to the so-called 60 days and 30 days rule."
Also no special drinks I think which would have been another disadvantage. Seemed to take it all in his stride, got to love Yuki.