Colin Johanson, GW2 Game Director,
"Rinith's reply is probably the closest description to the reality as of today where we stand with SAB. We've made a number of changes to the physics engine as a result of places like Silverwastes, Dry Top, and the expac that means we need to go back and re-work SAB to use all the new movement systems. It also needs a pretty solid rewards overhaul, and I know Josh would love to make some changes to world 2 before it returns.
We're all focused on expansion - both the announced details, and the un-announced details and simply can't peel anyone off a team to work on it right now without it effecting expac. The two SAB worlds we've seen will return as we've said before, but simply put our focus right now is on making the expansion amazing :)"
More Information:
http://dulfy.net/2015/07/01/gw2-colin-explains-why-sab-is-not-returning-soon/