Why do elite runners lose their mojo after an injury?

I think a lack of confidence is an issue. Also, a big factor is that the runners you mentioned are just getting older. It seems like, it's much easier to come back from an injury during the early 20s compared to the late 20s.

Loss of continuity, putting a few years of unbroken training together back to back to back...seems to be something that many elite coaches point out as a key to success.

Then, the struggle to reach previous levels...can be disorienting...discouraging...

Many are used to unbroken continued improvement year after year...which can be really inspiring and encouraging...

Bekele will make a comeback. His training was postponed because his new track was being built, and he's only done 3 track workouts on it. So long as he doesn't get injured again, he'll win gold. He ran that 26:43 as his second race of the year despite injury+dropout, which is pretty damn impressive. He may not be in his 2008 form (26:25 and blasting both Olympic fields) but there is also much greater competition this year in the 5 and 10 as Mo Farah and Rupp are solid medal contenders.