Injuries. His skating is a big weakness for him nowadays.
It doesn't seem like he has ever talked about it much, but the consensus timeline that we were able to piece together in Nashville was something like:
1. He was "slightly damaged goods" at the time the Preds picked him up from Montreal... Habs fans talked about some collision with Emelin iirc and maybe that was the start of a neck/back issue with him.
2. Nevertheless he was still a bona fide dynamic #1 defenseman in his first two years in Nashville: a tad slow out of the gate in Year #1 as he adjusted to his new digs, but overall our best defenseman those years, and best in the playoffs (and that's saying quite a bit with our blueline). Still, you could see he wasn't particularly fast in a straight line - good edgework, but if he was ever "fast" before in Montreal, then he lost that before coming to Nashville - he didn't have closing speed at all coming back on plays. But he still had such great vision and creativity and battled hard and played so well defensively that we didn't need him to be "fast".
3. In Year 3 in Nashville something had happened. He went from "not fast" to... "slow/immobile". Maybe he hurt something lower in his back, something went off a cliff for him physically anyway. He was still a pretty good 2nd pairing-calibre guy, he still battled, but his lack of mobility became a liability. I don't think we had any one specific on-ice event that we were able to point to as causing the issue. Sometimes we wondered if his off-season training for bulk strength might have been a contributing factor.
4. Subban did manage to battle his way through to a good finish/playoff that 3rd season so some of us were optimistic that maybe he would recover from the injury, whatever it was, and rebound the next year; but clearly Poile knew something we didn't. He got out of the contract while he could still get something, fortunately, because clearly whatever the issue was turned out to be chronic/permanent and it doesn't look like Subban ever recovered. Or at least, it would seem from the rare glimpses I've seen of Subban in New Jersey since the trade that he is basically frozen at that immobile level. Still tries to battle, but has given up even trying to do much on offense anymore, he seems to have settled into accepting a depth role where he tries to manage his physical limitations and just hold down a #5-ish type of job as he plays out his contract.
It's too bad, because he sure was fun to watch in those first two seasons as a Predator.