The separation of vanilla Minecraft, CraftBukkit, Spigot and plugins is what causes a lot of problems whenever Minecraft is updated. Since CraftBukkit and plugin developers do not work directly with Mojang (the developers of Minecraft), they have to wait for the update to come out first, just like all regular users. Thus, whenever Minecraft is updated, CraftBukkit may have a delay of several hours to a few days before they can even release an experimental (read: unstable) build. It takes a bit longer for a recommended (read: stable) build to be released. Once CraftBukkit has reached a stable build, that is when plugin developers will actually start to update their plugins to support the latest CraftBukkit properly since experimental builds of CraftBukkit change too often for them to keep up.