Is there a genetic component to success with the MAF philosophy?
It possible that I will still not see much increase in training speed at MAF heart rate because of some lack of genetic ability?
There is a genetic component to endurance performance capability. In other words, some people become elite performers in the endurance sphere because of their favorable genetics.
However, there is no genetic makeup such that endurance performance capability would be better arrived at by running at a high heart rate and burning sugar. There is not (and has never been) a single person on the planet for whom the system responsible for their health and endurance performance is the sugar-burning system.
People who have more powerful sugar-burning systems are essentially genetically tuned for sprinting and weightlifting (and to a lesser degree cutting sports like soccer and rugby). However, they still rely on their aerobic system, and specifically its ability to burn fats for recovery and the sustenance of health. It will never produce the energy necessary for elite-level endurance performance, but that does not mean that their more powerful sugar-burning system is now responsible for their health and their (more modest) endurance and recovery capabilities.
So, to come back to your specific question, it's not most people aren’t slow at the MAF HR because they don’t have the proper genes. They’ll never become incredibly fast at the MAF HR. That said, it's likely that the overwhelming majority (99%+) of people have the “genetic ceiling” necessary to run 7:30 minute miles at MAF (from their 20s to 40s). Whoever is running 13+ minute MAF miles isn’t because of their genes. This is far from the immense engine it takes to run Sub-5:00 MAF miles like Mark Allen or Galen Rupp, but it’s also more than a lot of people think they can manage.
Of course, to really hit your genetic ceiling, you might need to be living the life of the monastic athlete for quite some time, and be fully immersed in your athletic development. It might not be something that most of us have the opportunity to do given our commitments, responsibilities, and lifestyle. (ut that’s, of course, a different question.