in general - those that can be colonized are in the process of being colonized. - those that are not being colonized right now are either not worth it or too difficult.
planets that can be lived on are extremely rare and very precious. planets that are capable of sustaining human living and reasonable resources are reasons for large scale wars. in other words ... one cannot colonize / terraform a planet just like that. its a process that takes decades up to centuries of pumping money into it.
( in the past - a few mistakes were made, dishing out planets too freely ... also the only "really" professional organization with enough backup cash to terraform are houses )