Useful mutations become established much more quickly in a population of microorganisms, such as the cluster of e-coli bacteria shown here, than they do in humans. the most likely reason for this is that microorganisms

The answer is microorganisms have rapid cell cycle.

Microorganisms are much simpler and much smaller organisms than humans. Because of their size and complexity, they reproduce in extremely rapid pace. For example, E. coli population is able to double each 20 minutes. This is, of course, not the case with humans.
For the mutation to become established, there must pass a few generations. So, if a useful mutation establishes in E. coli cell, in 20 minutes it will be present in the next generation, in 40 minutes it will be present in the generation after that, and so on. In the humans, years (not minutes) are necessary for the mutation to be passed down to the first generation. It needs less time to establish a mutation in E. coli than in humans.