It is pretty odd. A few observations:
- There are lots of races in Mexico (just about every weekend in Mexico City) and they are popular, but mostly among the upper middle class hobby jogger crowd (think people who buy leg warmers and jog really slow in large groups), as races are often like 20-40 dollars, which is pretty hefty in Mexico, and outside the top 10 or so the races act as more of a social event.
- Obviously lots of talent gravitates toward soccer
- Many promising pretty good runners (not Diego Estrada or Juan Luis Barrios level) fade out because it's so hard to make a living (this isn't by any means unique to Mexico)
- There are some Kenyan and Ethiopian groups nearby and they'll usually clean up the paying races, which may have an effect (e.g. there are a couple big 10ks that pay like 500, 400, 300 for the first three overall, then maybe 100, 75, 50 for the first three per age group; three Kenyans will go win the overall in about 30min and split the $1200 among themselves, and while winning $100 is nice for a Mexican kid running 32min at 7,000 ft, it isn't enough to live and train on for the month)
Most of the population is in Mexico City, which is smoggy and doesn't have many places to run (there are, but you kind of have to know; the city doesn't lend itself to running), and sports in general just aren't a thing. In the US every kid does some sport in high school and most kids play sports through elementary and middle school as well, so talent is on display. In Mexico you either pay to play soccer in a private league, play soccer or basketball casually with your friends, go to the gym, or do nothing.
So in general, incentives and popularity. There are probably plenty of talented athletes working in other professions that pay better or doing other sports that pay better, and plenty who never had the Abdi chance where people realize your talent in this obscure sport so the guy doesn't even know he has a 28min 10k in him.