Mostly too poor and perhaps lacking better engines - we really need nuclear propulsion at the very least (with concomitant big amounts of radioactive material)

Mostly too poor and perhaps lacking better engines - we really need nuclear propulsion at the very least (with concomitant big amounts of radioactive material) to not spend a year in space just getting there, which could cause damage, physical and psychological. Or maybe some form of suspended animation. We cannot really build city sized ships, not yet. That'd require huge space stations. Self sustaining architecture is one thing, getting it scaled up is another - and relatively hard. Plus design so as to not cause problems related to living in confinement or in dreary space. This necessitates at least village scale if not a bigger city.