Because you should wait for the GOTY edition.
People who bought the original non-GOTY release were unable to purchase various DLC after they were pulled from the Store in favor of the GOTY edition, which they would be forced into buying, having no option to upgrade or buy whatever they were missing themselves. It was/is a pretty crappy situation.
You left out the part where the GOTY edition is considered a different game so your save and achievements don't carry over.
oh, you made it sound like there was something wrong with the game, heh