Luke's game is further ahead. All around further ahead of Nemec. His reads are quicker and he sees the ice better, his possession numbers are through the roof, his positioning is better, and his overall play is more refined...he's been playing 21 minutes a night for 51 games, that's more than he's ever played
People are seeing Luke hit a wall now at 50+ games, and comparing Luke's current slowdown to Nemec's current 30 games. People were calling Luke the best Dman on the team when he was around the 30 game mark too.
I'm not trying to take anything away from Nemec, his performance has been very good for a 19 year old. It just seems he has become a more of the flavor of the day due to Luke's more recent sloppy play that was inevitably coming late in the year after a heavy workload.