Likely technical; the last three major source games to be ported were Dota2, this and Counterstrike: Global Offensive (as yet not running on Linux), which were

Likely technical; the last three major source games to be ported were Dota2, this and Counterstrike: Global Offensive (as yet not running on Linux), which were their three newest games, and presumably used their shiniest, glossiest versions of the Source Engine. I'd imagine that CS:GO would have been prioritized much higher if it was purely a business decision, given it's still high up in their top-played games; Dota2, likewise, is their top played, flagship title, so if it was a pure business decision, that would surely be #1 instead of third-last.