Many reasons but one is money for equipment. Things like running and boxing have very low barrier of entry. While a healthy human can run anywhere for free (I g

Many reasons but one is money for equipment. Things like running and boxing have very low barrier of entry. While a healthy human can run anywhere for free (I guess preferably with some decent shoes) you need a modern very expensive bicycle to compete in that sport just as an example. Also a large middle class that has working parents who can support and spend money on a child to train and learn is a big part. Many poorer nations children young/adults need to be working and not training for the Olympics. Overall it's disposable income and disposable time being the crucial factor.