Simply put, Trump was threatened that if he pardons Assange, senate republicans (the establishment) would vote to convict.
I wish he had the courage to call their bluff, because them convicting Trump would fragment the GOP to an extent that is irreversible, IMO. Most people who aren't Trump supporters don't appreciate the extent to which Trump's base is not overlapping with the establishment GOP base.
"there was knowing and complicit behavior between the Trump campaign and Russians that stopped short of direct coordination, which may constitute conspiracy."[1]
1. https://en.wikipedia.org/wiki/Mueller_report#Redacted_report...
That's one of the reasons he colluded with a hostile foreign entity(Russia). Cause he doesn't give a shit. Sadly I don't have much hope for senate Republicans to do the right thing regardless of what Trump did, even after his role in the attempted coup
Trump doesn't have courage, he has bluster and self interest, he didn't see what was in it for him to pardon Assange