Part of it is how we take measurements on intelligence. Like everything else (height or weight), intelligence follows a normal distribution. Thus, there will always be a larger number of average people, rather than a large number of highly intelligent people. A reliable measure of general intelligence, IQ is also set so that the average IQ is 100; thus if on average everyone was intelligent as Albert Einstein or Stephen Hawking, there would still be more "average" people than very intelligent people.
There is some evidence, however, that under optimal conditions, the average intelligence was slowly increasing in developing nations. Either that, or IQ tests have gotten easier.
The Flynn effect is an observation of the substantial and long-sustained increase in intelligence test scores measured in many parts of the world from roughly 1930 to the present day. Ulric Neisser estimates that using the IQ values of today the average IQ of the United States in 1932, according to the first StanfordBinet Intelligence Scales standardization sample, was 80. Neisser states that "Hardly any of them would have scored 'very superior,' but nearly one-quarter would have appeared to be 'deficient.'" He also writes that "Test scores are certainly going up all over the world, but whether intelligence itself has risen remains controversial."