From steam store via reddit

Oops, this is embarrassing. Due to high demand we have temporarily run out of unique keys for this title. You may, by all means, purchase at the awesome sale price but may not be able to play until we receive more unique product keys. It shouldn’t take too long, we have sent armed guards to the publisher’s house to get them out of bed, drive them to their secret lair and produce more keys.

LMAO