In all my years of watching the NHL, I've never seen such an enigma of a player (and this includes guys like Kovalev and Yashin). Any possible answer is also contradicted by something else.
You can blame linemates but I know what it looks like to have a wildly productive player saddled with lesser talent (see Crosby, Datsyuk and early OV). These linemates might have trouble finishing or keeping up but elite talent can usually find ways to bring out the best in them. And this excuse also doesn't pass the eye test, especially when he plays with some of the game's best on the PP.
You might blame a possible injury but he doesn't really seem to be labouring, as much as he seems timid, confused and/or uninterested. When guys like Yzerman, Forsberg and Bergeron famously played injured in the playoffs, they managed to fight through it and you could still see the desire to produce despite the pain. People will point to Pettersson's wrist but then you watch him unload one-timers in practise and think "where's that been?".
Confidence? Well, maybe. But there have been moments where he's seemed downright cocky in interviews. And scheduling impromptu boat chats with reporters in the summer, to give thinly-veiled demands to management to ensure a winner, doesn't scream "insecure". Is his give-a-shit meter broken? Who knows? During one of his previous cold streaks, many were wondering if he was purposely dogging it to get Benning fired. I felt like that was conspiracy theory-level to suggest but, then again, his play almost immediately saw an uptick when Bennning/Green were canned.
Maybe we'll never really know? All I can say is that it is incredibly confounding and frustrating. He has so much talent and could be a top 5 player in the league. Hoping he can turn it around soon or the Canucks are racing to a red light this playoffs.