Because it's been superseded by Deathinitive Edition and delisted from Steam, thus making it unavailable for purchase. And since SteamGifts gets its price of points from US region and the game isn't available there - it's 0p.

However, old Steam keys still work and the game is perfectly playable.

They're separate games (unlike say the first game and its upgrade) IIRC due to how this has DLC while the DE has it included. In fact many entering want it for collection purposes.