Why did Obama issue a "stand down" order of counter-cyber activities targeted at Russia?

This.

As President, Obama has a legitimate role (in concert with Congress) in determining what United States’ foreign policy should be, and also the ultimate role in weighing and balancing competing harms and arriving at a compromise.

Obama had, in other words, every right to say, “This is bad, but upsetting the apple cart would be worse,” and issuing the direction in question.