Allied Race DKs caused the servers to crash on preorders. You're under-estimating Panda/allied race DKs lol.
Ever think maybe they want Naga in as the crutch midway between now and Shadowlands release to keep people subbed or to resub during content drought? So they wouldn't release that as an expansion feature if they want everyone in BFA to have it(especially since Shadowlands doesn't fit the plot of the Naga Allied race.