It's a little tough to explain without resorting to a level of math nerd that is hard to type out here, but the basic problem with it is that there's too much gene flow between demes (partially isolated intra-breeding populations), and that swamps the relatively weak effects of natural selection. It's part of the reason that almost all speciation is allopatric, because without that isolation, the effects of selection get wiped out by cross-breeding.