In the End Steam is about Money
Theyre not doing this for Fun
Good Game or Bad Game doesnt matter.
Only thing that matters is the Cash that Steam can rake in by allowing a Game for their Platform.
Games with alot of Media Attention will go in without Problems because Steam knows they will sell.
Same goes for Games which are backed by an Renown Company
But if you got neither of both you need to be attractive in another way
Small Scale Games usually do this by offering their Games very cheaply on Steam so Steam can distribute it as extra parts of packages or as bargain stuff
Bigger Games will close Contracts to be Steam Only in order to assure Steam that they will profit
Now Dawn of Fantasy wont offer Steam the sole Publishing Rights on the Game
Also Dawn of Fantasy comes from an young Company which doesnt have a Big name yet
And Dawn of Fantasy is also not an Smallscale Game.
So in the Eyes of Steam
Dawn of Fantasy aint Lucrative enough to be acceptet.
After all even if the Community would grow Steam would not get the Full Profit from it.
They would only have the Game itself which is a one time Download.
The Main Long Time Profit lying in the Item Store would not be on them
Well of course just a guess on my side ^^
I can only try to imagine how they tick
but I think it should be something close to this