IMO there's a few reasons. Last season he was playing 25+ mins/night some nights. This year his minutes are are down to probably more like 20/night. He has also missed some time with minor injuries (could be something nagging) and he missed some time early in the season at Flames camp.
But to me the major thing is there is no accountability from the coaching. He is trying to doing way too much out there. The team has one mission. Get the puck to Morin or Yoan Loshing and that's it. Teams have figured out it's not hard to defend and they've put way too much on those two guys.
I appreciate his compete level but he is a tire fire defensively. His decision making is hurting him, again not because he sucks but because he's trying to be Cale Makar out there. He needs to simplify things greatly and improve his skating. I don't see that happening at the junior level but I think he has the tools and drive that he'll take a step forward once he reaches the AHL.
I have a different read somewhat from you. Morin plays around 22 minutes per game which is probably down from last year, and yes Moncton's defense is much deeper this season.
Morin has been removed from the PP1 for long stretches this season, mainly (in my opinion) because he was forcing a ton of shots while the actual setup for this unit was to have Desnoyers find Loshing for one-timers, and since Morin really loves to shoot the puck from up top, he was moved to the second unit where he could be the main driver and offensive force.
I actually think Morin's offense has suffered a ton from him being much more focused on his defensive game (still not a strength by any means). Last year, I found he cost a couple goals to his team most of the games I watched him and was turning pucks over non stop trying to do too much. I find that he has cleaned that up to a very large extent this year.
This year has made me wonder just how much offense he actually has, I don't see a ton of vision or creativity, just a guy who can move well laterally on the blue line and with a very dangerous shot. Great puck skills as well.
If he's not going to be a PP QB at the next level, he isn't going to play. Defensively he's such a project.