Russell was a guy that was suppose to get you stellar defense and hit for average with maybe 10-15 HR's a year. That was his expectations. He's given you the defense with a bit of an issue when it comes to throwing errors but he's never reached that average hitter status.
So you are asking why he's been a disappointment? Because he's never reached being an average hitter. It's been 4 years now with no signs of improvement.
Does that work for you, sunshine?
On the vast majority of teams you could live with what Russell gives you. But the Cubs can just as easily move Baez to SS and put Ben Zobrist/Ian Happ/David Bote/Tommy La Stella at 2nd.