Based on what is Rielly better defensively? Burns had a negative GA/60 rel. He was +19 and nobody else on his team was even +12.
They may not, but they’re not a strong defensive team, and having poor shot and goal suppression metrics, relative to guys like Ron Hainsey, Jake Gardiner, Travis Dermott, etc. isn’t a good look. Meanwhile, guys like Erik Karlsson and Drew Doughty were peaking in seasons where Jamie Benn won the Art Ross, and never got a Lindsay nomination.
His competition was a healthy Crosby and a healthy McDavid. Those were the only things that separated Burns from winning the Award. (And we haven’t even seen the ballots. It’s possible that he finished ahead of Crosby.)