The Federation believes in non-intervention, and the right of a species to evolve on its own, to a frankly religious extent. They won't save a pre-spaceflight species from being wiped out by a plague, or an asteroid, or an internal nuclear war - and they definitely won't save a post-spaceflight species' oppressed masses from the conditions of their oppression. Depending on the writer this is presented as simple pragmatism on the part of the Federation, or the absolute peak of humanitarian ethics/the logical endpoint of believing in Freedom(tm). Personally I think it's a load of crap how far the Federation takes the Prime Directive, though I do admit it does create more interesting TV drama than just having the Enterprise use science magic to solve every problem in the galaxy.