What happened to bukkit?

Bukkit is still very much alive. Using either Spigot or Paper is a much better choice though, as they are more stable and offer far more customization options than Bukkit does. CraftBukkit however was DMCA'd. You can still download up-to-date versions of CraftBukkit on a website called YivesMirror. (Bukkit is an API that most servers use; Spigot is merely a fork)