Soft SF has been colonising planets for a century or more, but why would you do that in hard SF?
The simple answer is that I can’t think of any reason to do so. There’s nowhere in the solar system other than Earth where you could live without building your own self-contained environment, so why not just build a habitat in space instead? Mars is the one possible exception as it could potentially be terraformed to give an environment somewhat similar to Earth, but the effort required to do so would almost certainly be better spent on building vast numbers of habitats instead.
Outside the solar system the problem is even worse. The most realistic option for interstellar travel would be a ‘generation ship’ — but note that with life extension the same generation of passengers may survive to the end of the trip — and a fusion pulse engine; that would allow you to travel at perhaps 1% of the speed of light, so you’d be looking at several centuries to travel to the nearest stars. But if you’ve been living in a self-contained habitat for centuries, why would you then want to get off on a planet, so long as the habitat was still viable?
Note that finding a ‘habitable’ planet might even be worse. If we found a planet in another solar system with a dense atmosphere and life, there are probably good reasons to expect an atmosphere similar to Earth’s because oxygen is the easiest element for life to use and the concentration in our atmosphere is about as high as you can go without dramatically increasing fire risks; the concentration might be lower but probably no worse than living on a mountain. So breathable air and water should be fairly common.
But when we get to the life, that’s a different matter. The odds of such life being based on DNA or a similar molecule are probably fairly high, but almost certainly not in a human-compatible form. That should hopefully mean that Earth disease wouldn’t readily infect the local life, and vice-versa, but it may well also mean that Earth life would be unable to operate within the local food chain.
Overall, I’d say that you’d need some pretty special reason to justify colonisation in hard SF whereas in soft SF you can just pick the technology you’re going to use to support your story. This is one of the things that makes writing hard SF more difficult; instead of deciding the kind of society you want for your story and then picking technology to support it, you have to pick the technology and then figure out what kind of society that would lead to.