You think this is a coincedence?
They advertised something to make people want it. They realised the point at which most people would feel similarly to the way you do and put it on sale today.
Valve has piles and piles of money for a reason. They're not using a dartboard to decide on their deals or promotions.
(Re the people who don't understand Steam wallet drain: Steam provides the perfect platform to offer you a good deal on a game you intended to play, but never did. Since the deals tend to be short-lived, there's an incentive to buy it while it's cheap, which leads to essentially stockpiling games.)