Apparently Deadpool was pulled from the Steam store. You can still buy a Steam-activated copy from Amazon, Gamefly, GMG or Nuuvem. Gamefly still has it for $8 after coupon code GFDJAN20.
Supposedly, Activision lost their license to publish it on Jan 1, 2014. Since Steam doesn't actually generate keys for itself, they no longer have licensed copies to sell. The 3rd party vendors still have licensed, pre-generated keys.
Wow. Regular ol' Joph fan club in here.