Or perhaps because those other people who become social leaders had other skill sets not tapped by IQ tests. This idea is just about as old as IQ testing itself. Lewis Terman was the author of the IQ test that Marilyn of Ask Marilyn took when she was a kid, and that I took when I was a kid. Here's what he had to say about the limitations of IQ tests:

"There are, however, certain characteristics of age scores with which the reader should be familiar. For one thing, it is necessary to bear in mind that the true mental age as we have used it refers to the mental age on a particular intelligence test. A subject's mental age in this sense may not coincide with the age score he would make in tests of musical ability, mechanical ability, social adjustment, etc. A subject has, strictly speaking, a number of mental ages; we are here concerned only with that which depends on the abilities tested by the new Stanford-Binet scales." (Terman & Merrill 1937, p. 25)