This is pure conjecture, since LE hasn't addressed these questions, but I suspect that with 18 separate agencies (& now TES) involved in the investigation into HD's disappearance, LE has had enough professional resources available to conduct comprehensive searches of the area close to HD's home as well as any other geographic areas of interest to them.
With the amount of manpower and search tools at LE's disposal, including local, state and federal agencies, they made the determination that inviting the public's participation would have been more of a hindrance than a help.
Why now?
I think the answer to that is that LE has concluded that once TES completes aerial searches, they will have exhaustively searched the locations they had reason to believe were likely spots, and came up empty.
Unfortunately, searches like this can't go on indefinitely.
They eat up both $$$ and manpower.
The public is likely now being asked/invited to search because LE is at a point of scaling back their own search efforts.
The fact that no sign of HD has surfaced despite the extensive searches (sadly) leads me back to where I always end up in my mind's eye:
Water.
JMO and HIW (Hope I'm Wrong)