Why not now? Are they waiting to see if they change it back to Boy Scouts?
It gives time to react for those in the program. Charters for different organizations (Trail Life USA) can be put in place, credits transfered, new officers elected, etc.
I suspect it has to do with supporting boys already in the program, who are pursuing Eagle rank. With a waiting period of 6 months Life-Eagle, 6 months Star-Life, and 4 months First Class-Star, the worst-case drop-dead date to achieve First Class in an LDS unit would be the start of September of this year, so boys entering the program at the start of this summer could realistically get Scout, Tenderfoot, Second Class and First Class over the summer, and achieve Eagle by the end of the unit charters in a year and a half...
Usually that comes down to money. The Mormon Church sounds like they might be standing up a new organization. Incorporation and other legal matters would seem necessary.