Well, considering that it's a fictional universe, the reason why so many species have such similar technology levels is clear; it's more enjoyable to listen to stories about species who are close enough technologically that there's some narrative tension about who will win in a given contest. While you can tell stories about vastly more powerful empires (see, for example, Q) such stories are better taken in small doses; and Q never actually goes flat-out against the Federation, because if he does, the Federation will lose instantly and there will be no story. (Occasionally, the Federation has an effect in a Q-vs.-Q conflict, but that's as far as it goes).

I'm not entirely sure what you mean by "the Mysterious Space Police culling thing", though.