I don't think the US has gotten much weaker, nor has the rest of the world gotten much stronger, as a whole. Certain areas have gotten stronger, such as Australia and South Africa, as the talent pools there have increased.

I think the bigger reason is where these guys are playing. Naturally, when more of the top players from other tours are starting to play the PGA Tour, then they'll be taking up spots on the money list that probably would have gone to Americans before. The money and competition are so much greater now in the US that no one in their right mind would pass it up if they wish to be considered the best in the world. While 20 years ago a Sergio, an Ernie or Retief, or an Adam Scott would have played the vast majority of events on their geographic "home" tours, now they play considerably more in America. Hence the European, Sunshine and Australasian Tours are left out in the cold, their fans dejected and the only hope of returning some of the luster to their schedules is the farcical idea of a "World Tour."