All the chemical elements are available in space. Imagine a Drexler-style nanofab that can build anything atom by atom, given the right atoms. There's no compelling need to develop such a thing on Earth, but if we had it (because we needed it for space) the economic impact would be profound.
The enormity of the advances in technology that would be required to colonise space would inevitably transform life on Earth. And the amount of resources required need not be that great. There is no deadline, no hurry. We have plenty of spare capacity, 25% unemployment in some countries. The aim would be to bootstrap an autonomous colony building machine that lived off the land, not keep flinging megatons of stuff off the planet.
The main reason to choose Mars is PR. The Moon is old hat. The stars are a thousand years away. Mars is a clear step forward that people might get to see in their lifetime. Any large public project needs to have public support. Would a reality TV company contemplate funding a robotic mission to sniff a pebble near Uranus? How many people would tune in to watch the birth of the first Martian?