News flash. Rielly was never really that exceptional as an overall, even strength player. Sure, at 5v5 his offensive impacts, and transitional ability are great, but his defensive play has never, ever been good, despite some narrative that Babcock developed him into a minute munching, top pairing, shutdown defenseman by giving him tough minutes for one year or whatever.
Last season he hit a new level offensively, and combined with a bit of luck (shooting at 9% as opposed to his career 5%, to score more than twice his career high), people had him pegged as a Norris candidate defender just because suddenly the numbers were there.
Sure, overall, Rielly is a good player, a positive impact. The positives outweigh the negatives, but both are stark and obvious. He's not a balanced player, like say, Jake Muzzin, who doesn't particularly stand out in any area, good or bad. So, when he's playing with Ceci & Barrie, two guys who have the exact same weakness as him (preventing zone entries, passes to the slot, in-zone defensive play in general), he suddenly doesn't look so hot, and the weaknesses really stand out.
So, I'd say it is a combination of good luck last year setting our expectations too high, as well as Rielly's defensive woes being exposed by the fact that he's been stapled to Ceci & Barrie. Now, Hainsey was no true top pairing talent, but he was superior to both in defensive play, and I think Rielly misses that.