I'd say the main reason was that they weren't that good. At points last year, and the year before, I got the impression that the cup run may have been a fluke. They played above themselves and now they're constantly chasing that level. Sure, they get there at times, but have trouble maintaining it on a consistent basis.
And yes, I know that describes almost every eventual cup winner, but what's been more consistent with the Preds? Lack of scoring? Terrible discipline? It's a shame really, because Peks has pulled out 2 or 3 of the best years of his career and they've become unglued around him. He deserves better. (Though he's always good for one or two completely horrible playoff games).
I hope Duchene is the thing that gets them over the top, but I have doubts. Mainly coaching doubts. Sometimes change is good. That's directed at you, David Poile.