Even if they did spend the time and make a linux client for what they have they would have to work with each individual game and make it work with their platform. All that time and money for what? How many would run out and buy these games from steam now that they have them? Most people already have the native games they want so steam doing the work to port them now wouldn't net them much in the way of sales.
Now if developers start showing interest in linux as a platform they will obviously follow suit. Until then its a big waste of time, resources and money for them to port over a client for a small amount of old games.