Really ? Because Valve wants quite a bit of revenue from selling - which could drive the prices up.
Valve's known for often lowering prices and ESPECIALLY their crazy sales (Weekend, X-Mas, Summer, Mid-Week, etc etc).
I'd guess that Valve (Steam's 1st party publisher); the game's 3rd party publisher on Steam; and the game developers got to agree on when what goes on sale and whatnot.
