He's better than Martin in the offense, but clearly worse in the defense. And he isn't that good in the offense either to tolerate his lack of tactical knowledge. I understand Flick to go for the safe option, as we have enough firepower upfront usually. Especially his positioning is really bad, so he either fucks up the offside trap or can't defend a counter-attack because he isn't where he should be.
Flick sees this every day in training, so it's understandable he isn't giving him much playing time. It's sad, because physically speaking he suits of being a LB much more than Martin.