The reasons for the increase in USA born players are two-fold:

a) the maturing of (the much maligned sunbelt) US hockey markets – IOW, players who were kids when Wayne went to LA and Mario dominated with the Pens or even Modano became a Texas hero, are not all grown up and ready to play RJ UMberger is a perfect example as are Jack and Erik Johnson (no relation to one another or the mellow surfer singer)

b) the USA Hockey programme stepping up to identify, test and train talented players across the USA and then assemble them into teams who played together at international tourneys for years (rather than the former ad hoc approach of cobbling together players as an after-thought)