It's psychological to get you to spend more money than you normally would.
Here was an article that I saw on reddit earlier:
http://www.psychologyofgames.com/2013/07/the-psychology-behind-steams-summer-sale/
It's about the summer sale, but the same can be applied to trading cards, etc.
So psychology for the weak minded? I love Steam sales and will usually buy a decent amount of games but it saddens me that people fall for this crap.