I guess the problem, however, is that it's a large commitment/risk for the contractor to give up on their own business in favour of yours, right? Could you expand a little more on your experience with those CTO's, how you got them on board, etc.?
In my experience, it's always been down to what that person values. At my last company my "boss" was a contractor, and although he was given the ability to build a team however he saw fit, he didn't want to sacrifice his freedom. Before that, my CTO took the role because she believed in the team and the projects we had - and we did some fantastic work. The roles were almost identical, but the people were different.
My hiring experience is limited to developers, so I've never been a part of the actual hiring process, but I've bonded with a handful of people that were offered the role, and it's always been down to belief in the long-term vision and whether they'd value that role over what they value from their current role as a contractor.