Mutations are more susceptible to natural selection in large populations, such as the bacterial ones. The bigger the population it is and the shorter the lifespan of the organism, the tougher is the competition between the individuals. So, even a small change, a mutation that enables its holder to have even slightly more chance to survive and reproduce makes a huge difference. In addition, bacteria have prokaryotic cells that lack the DNA repair mechanisms and they are replicating more often than human cells with eucaryotic cells, which explains the higher rate of mutations in their genomes compared to human ones.
