While WGT and the USGA have an agreement, it seems WGT has a separate agreement with Pebble Beach Golf Links which is why we have had Pebble Beach Golf Links available for CTTH for quite some time.
Earlier this year WGT announced PBGL had renewed their agreement with WGT and expanded it to include PBGL as an 18 hole stroke play course.
Shortly after that the team of photographers (WGT has said it takes about 2 dozen of them) descended on PBGL to do the needed photography.
WGT has noted that it takes 200,000 to 300,000 photographs to provide us with an 18 hole stroke play course. The photos get edited, stitched together, and married to the software that defines the topography of the golf course.
From start to finish WGT says it is a 6 month or so process.
Bandon Dunes for 18 holes of stroke play was started before WGT and PBGL renewed their agreement, so it's likely we will get Bandon Dunes before we get PBGL.
I believe WGT delayed Bandon Dunes and PBGL so they could devote time to addressing a range of issues the game had.
So, we get 'em when we get 'em.
WGT has a time limit for making Chambers Bay available in time for the 2015 US Open. We usually get the front 9 of the next US Open course for CTTH play in March.
We already have the 2016 US Open venue - Oakmont CC. That will provide WGT a space of time to add an additional non-US Open course in 2016.