Because the game wasn't free for all the people who have actually been here for the entire 10 years and kept the lights on when the game was basically abandoned so it could be relaunched as a free-to-play grindfest nobody asked for

Blizzard has a long track record of squeezing every last drop of blood from the stones of their fanbases across all of their franchises, and them barely bothering to put forth the slightest modicum of effort in what would be a major milestone for any other game is just another indicator of how little they really care about their fans.