And this is amongst the reasons that track won't ever get popular. It's not just Mo but he is the latest example of a champ who doesn't go against the best with the exception of when required in a world champ type race.
You see (or at least used to see) it a lot in the 100/200 races where the top guys avoid each other like the plague. As for the marathon, nobody realistically expected him to win in his debut so even though it was London with the big names, it's an easy escape but good for him for trying that one.
Mo may not be the only one who does this but it stops the sport from being really popular and stops the individuals in the sport from being popular. Hats off to guys like Rudisha who will step it up against whoever, whenever.
Look at other individual sports like Golf, Tennis and race car driving. Top guys in golf and tennis are in the same tournament a good number of times per year. NASCAR is on a weekly basis. Track doesn't get this in most cases.