Here is the graph.
The biggest trend is one that has been talked about a couple of different times here, and that is the decline of hockey players from Massachusetts. While Minnesota and Michigan are on the rebound after a slight decline near the turn of the century--during the NHL's europhilia phase--Massachusetts has been in a steady decline, and as Mirtle points out, with vets like Jeremy Roenick, Keith Tkachuk, and Bill Guerin near the end of their careers and few promising reinforcements on the way, those numbers should continue to decline.
The most common theory for why it be how it be is that the Boston Bruins have almost completely fallen off the sports radar in the city of Boston. The good news is that with the recent creation of a salary cap, the NHL has become egalitarian to the point that even the penny-pinching Bruins are able to compete, and if there is one thing the city of Boston loves--besides white people-- it's a winner. I mean, their baseball team somehow gained sovereignty after winning a title. The Bruins currently lead the NHL's Eastern Conference and a couple years of playoff success could change the sporting landscape there.