There are a couple of asian football players, aren't they? But what about women? No women play in the "proper" leagues do they?
There is something quite simple, that is regarded as very politically incorrect to say: in almost any activity there will always be some ethnicities that are, on average, better, and some that are worse.
Take basketball. Is it advantageous to have a very tall team or a very short team? A tall team. So, which team would have an advantage: 6'7 Massai, or 5' pigmies?
In rugby you want to have huge forwards. So, which team would have an advantage in a scrum: a Western Samoan team made up of chunky Jona Lomu-alikes, or a team made up of slender Vietnamese chaps?
Now it is wrong to say that any advantage holds true in ALL circumstances. The average Japanese man might be smaller than the average Anglo-Saxon, but I definitely wouldn't want a cross Sumo wrestler sitting on me.