>Why is POTUS holding back DECLAS?
Because it will inevitably damage the relations to foreign countries/ foreign policy, if the public would know that other governments and institutions plotted against Trump. It is one thing to "know", but a whole another thing, to make these things public.
Imo, he's threatening to DECLAS, in the hope that certain parties will be compliant and stop.
He for sure isn't fond of having to DECLAS, to achieve the same goal.