Deadpool only lasted six months on Steam before it was unceremoniously removed at the beginning of 2014. Activision's licensing contract had expired, which prompted Marvel to take several of its superhero games off digital storefronts. It's a pretty common occurence, but it was unusual in the case of Deadpool because the game was still kinda new.
Something has changed though, because Deadpool is back on Steam. No announcement or press release was distributed to celebrate its return (the game was pretty average, let's be honest), but it's right there, and you can buy it right now. With the Deadpool film adaptation on the horizon, it seems sensible to have it available. Surely someone can make money out of it.