Mainly because most raiding guilds including mine, had a lot of their normal raiders swap to DKs when WotLK came out. DKs are great, but if you already have 2 or 3 in your raid you don't need anymore. That being said. My DKs have started to become alittle inconsistent due to summer.
Maybe thats normal or maybe its because the people who swapped classes are more likely to burn out, I cannot say.