When a server's population bunches into the same level bracket, the hunting areas for that bracket become unusable. The Dimensional Desert exists to give that crowd somewhere else to go.
The level band moved
It originally covered levels 49 to 70. As server averages climbed, the crowding moved with them, so the zone was re-tuned to 68–88. On Knidos that leaves two Dimensional Deserts at the same 68–88 band rather than one of each.
Getting in
Talk to Retired General Hasim. He stands inside the bounds of Bandit Fortress in the Jangan region, at the Taklamakan entrance, and he is the way into both deserts.