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Why there aren't any serious proposals to ban space colonization?
That is, successful attempt to establish a colony will most likely create society that blames Earth for their misery, and "self-sufficient" colony probably requires nuclear technology (Zubrin's plan states this explicitly). They will have both motive and means to nuke Earth for good. Colonization greatly increases extinction risk, contrary to what space advocates say.
If the reason is like "that is far-future problem", why it does not work for things like nanotechnology (there are organizations that want ban it right now)?
If history is any indication, separate cultures tend to end up fighting each other if they want the same resource. Whether space colonies end up in such a situation is unclear, but seems unlikely. There are also religious reasons one culture would try to convert or remove another, and that's a bigger worry. Hopefully establishing hundreds or thousands of colonies would mitigate this risk, since diversity tends to help to stave off extinction.