Halo was XBOX-exclusive because Bungie was owned by Microsoft; their specific goal was to make an XBOX-exclusive title, and there was a very real incentive for them to funnel energy into the platform. DICE (and to a much lesser extent, Valve[1]) gets nothing specifically from having a Linux version besides the slightly larger target audience (and PR); there's nothing in it for them to spend massive development time on a AAA game and not release it on Windows + Mac + Consoles as well.
Am I missing something?
[1] From what I understand, Valve's pursuing non-Windows options to try and avoid a shootout between them and Windows Store. That's pretty understandable in of itself.