A world revamp would likely have been worked on in the background for ages. The assets especially we know is being updated slowly ever since MoP, and BfA showed that revamping zones goes by fairly quickly when it's a half and half approach like Darkshore compared to Arathi.
Something like Dragon Isles meanwhile likely required loads of custom assets, entirely new zones and generally more work to make it from nothing to what we expect.
It would likely also require a more holistic approach than just having zone designers fiddle with it in their spare time.
In short a world revamp is more conducive to being made slowly over a longer time by mostly independent artists.
Dragon Isles would require far more intercommunication between zone designers, and would likely therefore have to be made in one big chunk by dedicated staff, and not just what a couple of designers fiddle with in-between making Shadowlands.