It may not seem that way, and he may not act that way, but President-elect Donald Trump actually has some powerful reasons to support a full inquiry into what Russian hackers did and didn’t do to interfere with this year’s presidential campaign.

He has two reasons in particular. The first is that a cloud of uncertainty will hang over his election as long as the charges that Russia somehow intervened to help him remain unexplored and unanswered. Mr. Trump may fear that an inquiry will somehow delegitimize his victory, but the...