In the latter part of 2015, Microsoft slowed up on accepting new testers in to the Xbox One Preview program. Not much has been said since, but the questions have continued and now, Xbox's Mike Ybarra, has clarified the current state of affairs.
"Lot of msg on this so answering broadly: we have a lot of folks in queue to get into Preview on Xbox One. Still TBD when and if more needed. - @XboxQwik"
The Xbox One Preview Program has, in the past, been pretty generous when it comes to accepting eager gamers willing to try out the latest features before Microsoft pushes them out to the general public. That, in turn, led to the program reaching near capacity, hence the invites slowing up to almost a complete standstill.
Ybarra's words don't give us much hope that things will change any time soon, so if you're not already in, you probably won't be any time in the foreseeable future. Nevertheless it's always good to get a little clarity on such matters.