In Rio, the American Marathoners Ran Faster, Virtually Everyone Else Ran Slower - Why?

Paula of Brazil (15th) was not having a good time  

At the recent Olympic marathon, the two American-born athletes ran PRs, beating their previous best times by about a minute each. Galen Rupp, finished 3rd in 2:10:05 (previous PR: 2:11:13) and Jared Ward finished 6th in 2:11:30 (previous PR: 2:12:56).

Every other top ten finisher ran slower, sometimes dramatically slower than their PRs. Elide Kipchoge of Kenya finished 1st in 2:08:44 (5:39 slower than his PR of 2:03:05). Feyisa Lilesa finished 2nd in 2:09:54 (5:02 slower than his PR of 2:04:52).

The next ten finishers included 5 sub-2:10 marathoners. None of them did that in Rio.

Two Kenyans and one Ethiopian failed to finish in Rio. They had previous scorching PRs of 2:06:13, 2:03:51 and 2:04:24.

Before the race, on paper at least, it looked as if the Americans had no chance. But virtually all of the top runners had disappointing performances in terms of time.

Except for the two leading Americans.

Consider:

What explains this?

I really have no idea. But let's look at three possibilities:

1. The humid conditions were brutal. Only the Americans were ready for this. Undoubtedly, the Americans trained very well (more kudos to Rupp's coach, Alberto Salazar). But all the top runners are relatively wealthy professionals with top coaches who, among other things, anticipate the possibility of varying race conditions. Most of these runners train on multiple continents. This explanation does not seem satisfactory to me.
2. The pace over the first-half was slow. Thus, PRs were not in the cards for most runners (the reason Rupp and Ward ran PRs was because their PRs were relatively slow coming in). Indeed, I think each of the top 10 finishers ran a negative split (their first half was slower than their second half). But of course, this doesn't explain why the initial pace was so slow. Rupp claimed that his strategy was largely just to hang on to the favorite, Kipchoge for as long as he could. That sounds like a pretty good strategy to me, if you're Rupp. But why did Kipchoge and the other East Africans set such a slow pace at the beginning? It strikes me that if you are 7 or 8 minutes faster than, say Rupp, you want to take advantage of that over the full length of the race. The last thing you want is to have a 10,000 meter champion still with you with 10,000 meters to go. I'm not claiming Kipchoge and the other speedsters did the wrong thing. Obviously, they're smarter than I am when it comes to pacing. But I simply do not understand their strategy.
3. The Americans were simply hungrier than the others. The Olympics are now the one international race where there is no monetary reward. All the other top runners (outside of the Americans) have successful careers where they make hundreds of thousands of dollars by finishing well at other races. So, perhaps from the point of view of one of these professionals, if things are not going perfectly at the beginning of the non-paying Olympic race, why kill yourself by going fast and thereby perhaps messing up your performance at the next paying marathon? Indeed, this might explain why the three super-fast East Africans dropped out. (My point is not to be critical, only to understand.) But again, this doesn't really explain why the fast guys didn't make more of an effort to set a faster pace at the beginning, or why more of them didn't try to hang on for a Silver or Bronze. Even the pace in the second half of the race was slower than many of them had run before. 

Again, my intention is not to be critical of anyone, least of all the superb East Africans.

But I do think it's a puzzle.

Does anyone else have any better explanations?