To me, there are two reasons we're doing it too soon;
- We don't really have technology needed to build a self-sustaining colony anywhere outside Earth; say, a colony on Mars is inherently dependent on Earth's supplies, and will quickly die out as Earth does too; the technologies needed can largely be developed on Earth;
- The chance of some asteroid obliterating Earth in the coming millenia is so minor we might as well focus on much more real threats.