Has got to be either low priority or technical limitations. I guess a Linux port of Portal 2 isn't likely to provide returns anywhere close to PC sales, so it may not have been approach furiously by Valve.
I also wouldn't be surprised if there were some DirectX specific stuff in there that wouldn't easily make the transition to OpenGL either. Not to mention all that driver testing they would have to do...
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.
I'm not going to pretend I know when CS:GO was ported, but for a very long time (until the crate update) it was easily the least played version of Counter Strike.
I think it was ported to Linux before it was successful, but I can't be sure.
Since it had an OSX port already I don't think it would be anything DirectX left. I'm betting it was expected sales and/or driver issues on linux. I know it's been recent that some of the features I think they're using for portal 2 were supported in open source drivers.
