A post on Dear Esther and Unity from Robert Briscoe describes the reasons he is working on porting Dear Esther to Unity from the Source Engine, even though the game was first released almost exactly two years ago (thanks nin via Joystiq). This involved unexpected middleware costs, difficult ports, and concerns about the ability to support the game going forward. Here's how things came to a head:

When the PS3 port collapsed, I realised that with my knowledge of Unity, there was an opportunity to not only safeguard the future of Dear Esther, but to also clean up the Linux and Mac ports and reach a wider range of other platforms. Best of all, we’d be able to keep everything in-house, at low cost, with no more licensing or communication barriers, no more support woes and no more scouring for experienced Source Engine developers to help us.