cannard: I don't know about Bloodlines but I think it may have something to do with the fact it was made with the Source engine. Not sure if Valve would be too permissive about a Source game being sold through a means outside of Steam, even if both the publisher and developer are on board with GOG.
Wishbone: Dear Esther runs on the Source engine, as far as I know, and it's sold outside of Steam.
I know it's sold on online shops that sell Steam keys... has it been released, say, DRM-free? I know it was part of a past Humble Bundle, was it then?
I dunno, what exactly are Valve's rules regarding distributing games using the Source engine anyway? Someone here ought to know more than I do about this matter.