Maybe I can answer this, I am a mexican, been living in mexico city the last 7 years and actually was a college athlete;
First, track and field is a famous sport, but in Mexico all the sponsorhip efforts, all the interest and the main spotlight goes either to soccer or american football.
Another thing is that there is not a serious sports institution in the country, or at least it doesnt operate as it is supposed to. The thing is that there is plenty of young talent among mexican runners from different ages, but the corruption present in every stage of the institutions block these talents to emerge in a serious way, and if someone does, its costly because there is not an incentive from the government, or from anyone to try and succeed in a more professional way.
The last thing is overtraining or racing too much; in the last years running has increased a lot in popularity. That’s not a bad thing itself, but every weekend there are a lot of popular races which give good prizes to every age group. Because of that, anyone who is good or above average dedicates only to train to win these road races, not thinking about succeeding in a real, professional level.
I hope I helped with the question