Pricing leverage is a factor, but the major thing for me is that I usually don't like working with a client for more than 3-4 months. If you go beyond that time frame (in my experience) you usually see dependencies between the freelancer and the company cropping up, which I want to avoid as much as possible, as that often leads to the expectations of either party to be hurt sooner or later. This also puts me in a position where I can transition to a more interesting/lucrative client very quickly, with the old client still being happy.