I haven't played through this game entirely, but I assume it's because the skyfall mission is a separate DLC that isn't included in the base game. To my knowledge, the only legal way to get it is to buy the game on Wii U since the skyfall mission is on the disc in that version. But for 360, PS3, and PC, I don't think you can download it unless you downloaded it before they took it off the store. I may be wrong about that though. @InfiniteSheldor probably knows more about that than I do.

Didn't know about PC including it on the disc, but yes, it's considered DLC because PS3 and 360 users have to download it, and therefore isn't required to finish the base game. Wii U releases have it on the disc but probably isn't the fastest console to run on if you can help it.

@FitterSpace although the game itself was taken off the store, the DLC is still up to my knowledge (it is on the European PlayStation store, at least), presumably because while the games themselves are still available in physical copies, the DLC is not, and luckily their other Bond games had no DLCs.

Two years later...

On PC you can get only Skyfall DLC, it will download automatically with the game