> hunger, poverty, education and incurable diseases?
Those are threats to individual human beings, not to mankind as a whole. A Great Plague or a (non-nuclear) world war here and there can wipe 1/3 of humanity, but for the species as a whole it's just an inconvenience: it can be compensated in a couple of generations, through our adaptable fertility rate.
There are dangers, mostly ecological, which can wipe out the whole species: nuclear winter, mega-volcanoes, meteors... (yes, most of them are about some form of climate change) The mitigation for those dangers is to have several separated ecosystems, not regrowing the population locally after a demographic setback.
A city on Mars would probably take centuries to turn into an independent ecosystem (including all of the mining, transforming and recycling of every material required, and the means to extend itself across the planet. Or even, maybe, terraforming). But a partially Earth-dependent Mars city is a very important milestone towards the big goal.
Finally, he doesn't say that all of our efforts should be dedicated to that single goal: he's quite involved in fixing Earthly problem, including energy, climate change and transportation. He's also vocal about how we should think very hard about the kind of IA we want to cohabit with.