I don't think that has ever been the case.

Physical prowess was probably a lot more important in prehistoric times than it is today in deciding social power. But humans have a complex social structure, in which all kinds of social skills and intelligence matter to advance. A good leader of even a band of twenty hunter-gatherers needs to be socially smart. He needs to be able to make decisions that are accepted by the group and communicate them effectively.

One reason why this "strongest one = leader" idea misses the mark is because it is based on a misunderstanding of the origins of mankind. Homo sapiens' closest relatives are not gorillas or orang utans, but chimpanzees and bonobos. Gorillas and orang utans have troops with one dominant male whose rank can remain uncontested for years. Chimpanzees and bonobos live in communities in which the most dominant male must rely on allies to keep his position, which he can and often will lose at any given time if his allies are unsatisfied enough and his opponents strong enough.

In my opinion Frans de Waal's work on Chimpanzee social behaviour is very helpful in understanding the fundamentals of human social power structure.