I'd say it's quite obvious why League of Legends isn't on Steam.
Gemini Rue isn't on Steam because they wouldn't accept it for whatever reason, I don't remember exactly.
Add the Command and Conquer games. They don't have any of them from Generals back. There are actually a metric crap ton of old games, that will just never see the light of day on DD services. Which is a dam shame.
I'm really shocked that neither the Icewind Dale nor Baldur's Gate series have yet to make it to the platform. You think it would really suit the target demographic.
I for one am a bit miffed that neither of the Homeworld games are good to go up there. A good deal of quality strategy games are on the service, but these two don't make the cut...odd, considering Relic makes the Dawn of War games, and they have almost all of those up there...