I am sure once the content goes live SE will tell us.
I'm just speculating but... in past expansions, there was a feeling of discontent when an expansion was released and most of the players couldn't access the servers due to heavy traffic. This lead to a small group of players getting to the expansion areas 1st (exploring them and amassing items/info before anyone could). They then used it to their advantage by getting to the "good areas" and camping them well. This left a sour taste in many player's mouths (if I remember correctly).
I am betting SE is waiting to release the content only when the server traffic stabilizes again. This way, everyone is patched and ready to go. Everyone would have an equal opportunity to explore the new areas at the same time.
Edit: Servers are fine now. Download time (for the PC at least) is normal. Expansion content is live.