From their blog...

This 1.5 release will be the last version of Boxee for PC/Mac/Ubuntu. It will be available on Boxee.tv through the end of January.

We believe the future of TV will be driven by devices such as the Boxee Box, Connected TVs / Blu-Rays and 2nd screen devices such as tablets and phones. While there are are still many users who have computers connected to their TVs, we believe this use case is likely to decline as users find better alternatives. People will continue to watch a lot of video on their computer, but it is more likely to be a laptop than a home-theater PC and probably through a browser rather than downloaded software.

To our computer users… to those who have come out to our NYC and SF meetups, talked with us at Engadget, GDGT, and Giz Gallery events, or enjoyed Austin BBQ with us during SxSW, or simply messaged with us on Facebook, Twitter, and our forums… thank you for all your support – we would not be where we are today without you. But we can’t stay here.

We are excited about the upcoming release of Boxee Live TV and our new product roadmap for 2012. If you are a current Boxee user on a computer we hope that you will enjoy 1.5 and maybe when you are ready to retire that good ol’ HTPC/Mac Mini you will decide to get a Boxee Box.
Not a huge loss, since we will still have XBMC (which Boxee was based upon), Plex and Windows Media Center.