It looks like Bioshock 2 can't be purchased from the Steam store anymore for reasons unknown, but according to 2K games they are working to solve this issue.
A representative from 2K Games told PC Gamer today that it "will return to these digital stores soon."
BioShock 2 has been temporarily removed from the Steam, Xbox Live, and PlayStation Network marketplaces and will return to these digital stores soon. We'll let you know when that happens. In the meantime, this in no way affects the single-player or multiplayer experience for those who currently own BioShock 2.
The change to Bioshock 2 hasn't affected the original game or Bioshock Infinite and it isn't just Steam that the game has been removed from, as Green Man Gaming have done the same thing. With no definite reason it is hard to say what the issue may be.