The question was brought forward on the official forums of implementing some realms that have no Burning Crusade content. Blizzard poster Drysc explains why they have not done this:
The idea of having non-expansion realms has definitely been brought up before, but it's not something we want to pursue currently. We don't want to break up our player base by what they have or have not bought by entire realms. We like having players still able to interact even if they can't visit all of the same places, and we have attempted to encourage it by making sure players have to go back to Azeroth every once in a while.
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