Luke Schenn was never good.
He had the advantage of playing in junior with a man's body and could dominate the opposition based on being bigger and stronger than most everybody else out there. Couple that with being part of the Hockey Canada hype machine and a "cheerleader effect" of graduating with a stellar Canadian class including Stamkos, Doughty, Pietrangelo, etc. and being drafted by a team desperate for a fresh start, you had the makings of one overhyped, undertalented defenseman.
He never developed much in the way of offensive ability, handles the puck like it's a live grenade, has a terrible wind up and an inefficient point shot, passes like he's in a fan contest, has lead for feet, is too slow to throw big open ice hits, gets cycled like crazy, doesn't seem to have good defensive positioning, whiffs on the puck often when he's trying to make routine plays, an crumples under pressure. He can't fight, doesn't have a natural mean streak, etc.
He's just really bad.