Everything the community built up to that point became useless after the changes that were applied. Texture packs were changed from one giant picture to separate files per block, forcing every artist to update their packs or get left behind. The texture pack customizer was forgotten about because of that change also.
The largest change was when single player was incorporated into multiplayer, which completely destroyed the modding community. Servers became terrible to maintain, especially RPG servers with ongoing stories, when things were updated every few weeks. I know the bukket and forge communities sprung up after that change, but the original and influential group of people that were brought up with that original idea of Minecraft started playing other videogames that scratched the same itch. Sup to those guys, we all miss you here!