Why did dinosaurs evolve earlier than humans?

This isn't really a "why" question; but dinosaurs were not here first - for thousands of millions of years the stromatolitedominated life on Earth. In geological terms, the dinosaurs came along fairly recently.

One reason why large mammals (such as ourselves) did not evolve until after the end of the cretacious brought the end of the dinosaurs, is that there were few environmental "niches" - large mammals would have to compete with large dinosaurs. It was only with environmental changes causing the mass extinction at the end of the cretacious that mammals were able to survive where the dinosaurs could not (at least in their existing forms, there's good evidence that modern birds evolved from dinosaurs).