Have you looked at the shattered Luclin moon lately?
Look at it every now and again.. the halves are moving closer together, I kid you not.. Also, if you do the 'Staff of the Observers' HQ and talk to Vhizz Frugin, there's one part where he says 'Can it be? A snakeman on the moon?' which would be the Shissar on Luclin..
Think about it.. /nod
So I agree with the poster who said so they can spread them out over more expansions..