You would need space docking and refueling. What would the propulsion system be? It would need to catch asteroids early enough that small changes in their orbits would translate to clean misses of earth. You can't do that in low earth orbit!
But just think of what it would mean to have a craft that could flit around the inner solar system with enough power to nudge asteroids! Just that technology alone could open up space travel!
Space travel is already open. Space development is not happening, and the Treaty is the sole and only reason there is no private devleopment of space resources right now.