Because he knows that if he breaks the story, the rest of the MSM will have an excuse to ignore it, which they probably will anyway.
It comes from The Hill, which (I believe) is seen as authoritative by the worker bees in the MSM. Therefore it has at least a theoretical chance of not going into the memory hole.
Hannity knows this, and therefore stood aside. The "tick tock" thing lets everyone who cares about such things know that he had advance knowledge before anyone else that the story was coming, presumably from his Friends In High Places.
I guess if Hannity remains silent then his tweet will never be completely understood as to its true meaning. If Assange never releases anything the same will apply to him.
I am still thinking that Hannity's & Assange's tweets are related.
Not saying the thought of this being what Hannity's tweet was about is wrong, just that I still believe Assange is getting ready to release something besides just the deadman's key.