That's just two [species of chimpanzees]
There is a huge behavioral gap between the two species. I get your point that the human lineage had several, not just two, forms. Yet these two chimpanzee species are so brutally different that they make most of our "human diversity" seem so regularly the same.
About your question - it seems obvious to me the point brought by @Pete: That we, humans, came from the forests to the open land, which brought the seas and continents to us. Many different habitats would lead to wider variation.
A chimpanzee fossil that is 2 millions years old should be pretty
different from a modern one
Why should it? The African Forests changed drastically in the last 2 million years? If not, then why should chimpanzees have changed? Take a look at the horseshoe crabs (Xiphosura, Limulidae): they've been that way for hundreds of millions of years! The punctuated equilibrium seems to be the norm, more than the exception, doesn't it? (I remember the horse evolution through the Mesozoic, apparently linear, but in a closer look there ain't some teeth in the line? Aren't there plateaus - long periods of stability - among the high precipices of sudden genetic/form/function change?