Bacteria evolve faster than humans because

A colony of bacteria (say A) growing on a given medium has built-in variation in terms of the ability to utilise a feed component. A change in the medium composition would bring out only that part of the population (say B) that can survive under the new conditions. In due course of time, this variant population outgrows the others and appears as new species. This would happen within days. For the same thing to happen in a human it would take millions of years as life span of humans is in decades.