That’s an assumption. The intelligence community is very smart. You assume that we know all there is to know, but maybe that’s what you want to believe. The leaks that have come out, so far (and which, taken with tRump’s cloying, subordinate behavior toward Putin, I find very compelling), may well have been strategically timed to gradually erode tRump’s rabid support.
Such a maneuver would be wiser than revealing the smoking gun to an irrational public that does not want to see it.