At the end of Trump's first term, he was about to get impeached, and a controversial pardon could have gotten him convicted.
He was impeached during office - twice - and the reasons had nothing to do with Snowden... not even remotely:
- abuse of power & obstruction of office (#1)
- incitement of insurrection (#2)
impeachments are political. Trump making politically unpopular pardons would erode his political support in the senate. So there may have been more republican senators willing to convict.