Clones don't work well with the evolution. All of the individuals are exactly the same, so they are very prone to a whole population being wiped out at once. The key of evolution are mutations that promote survival of the best adapted. And the bacterias are different from what I understand because they reproduce at extremely fast rates. The more complex creatures could do the same but it would take centuries to do the same number of cycles and they (as a population) just wouldn't survive that long. Mixing genes is thus a better way for bigger creatures.