I was working with many Indians a few years ago and they asked themselves the same question. One had an interesting take I've not heard elsewhere - he claimed that the large number of vegetarians in India results in poorer athletic potential - the lack of meat and protein leading to less strength, quickness, etc.
It was an interesting idea, but looking up some facts myself right now, that claim could be tested by Pakistan's medal count. Same general genetic background, same general infrastructure problems, but are not generally vegetarians. However, Pakistan hasn't won a medal in 20 years, and only a few medals ever.
Kenyans and Ethiopians tend to get even lower levels of protein in their diets and their Olympic athletes are pretty damned quick. As running sub 3 minute kilometers back to back for a whole marathon.
Their macro-nutrient is generally about 10%-15% fat, 10% protein and 75%-80% carbs, and mostly from plant sources.
Was Dave Scott a vegetarian his entire life, or simply converted to it when he was a fully-grown adult? Most likely the latter.
Protein consumption as a growing child and teenager is directly proportional to height - it's the sole reason the average white male has increased 6" over the past 200 years. Genetics haven't changed, just our diet.
Dave Scott was awesome for sure, but what matters here are the Olympic sports and each one requires certain characteristics. You won't see a weightlifter competing in canoe, 5000+ running/walking or shooting for example.
Anomalies aside, it would be interesting to observe correlations between a largely ( >51% ) non-vegetarian diet and athletic predisposition and performance.
Holding every other factor (race, geography, culture etc) constant, the exponentially greater protein availability (in meat consuming countries) cannot NOT have consequences for general muscular build among the populaces and as a consequence, some form of athletic predisposition.
A good question to start with:
Are there any predominantly vegetarian regions - besides large tracts of India - elsewhere in the world?
It was an interesting idea, but looking up some facts myself right now, that claim could be tested by Pakistan's medal count. Same general genetic background, same general infrastructure problems, but are not generally vegetarians. However, Pakistan hasn't won a medal in 20 years, and only a few medals ever.