Mafia 2 has been removed from sale on Steam and other digital distribution outlets. There's no official statement as to why the game was withdrawn from sale but we've seen the same thing happen in the past with games like BioShock 2 and Deadpool so we can probably take an educated guess about what's going on. It was likely removed due to licensing issues, the game featured a lot of licensed music and even collectible vintage Playboy magazines, and one of those licenses probably expired and couldn't be renewed in time hence the stop to sales.

If our above assumption is true it's likely we'll see the game return to sale in a week or two, though in the recent case of Deadpool the game was gone for months so it might be best not to hold your breath.