My answer (by no means do I claim it is the right answer) is the evolution of football through athleticism and technical ability.
Suppose a new sport is invented that no one has ever played before. Who will be the best at this new sport? Well, most likely, it will be other athletes. Athleticism, such as quickness, strength, speed, awareness, coordination, balance, etc will obviously play a major role. So the first attempts at this sport will be dominated by the biggest, fastest and strongest. But after some time, the losers realize they must develop a bit of strategy. Tactics become more important. Those who aren't the biggest, fastest, or strongest will find ways to compete, develop new insights, and change the style of the game. The athletes may adjust to these changes, or they may not. The weaker side's tactics and technical ability will grow faster than the athlete's because their game depends upon it. The athletes can jump higher or run faster so they do not rely on technical ability. Yet the smaller players must, in order to compete, develop a better understanding of tactics and become more technical. (Can you say Lionel Messi? And how about Clarence Goodson? If you said "who?" - that is exactly my point.) And so the gap begins to shrink. Now after some more time, years, decades, generations even, all the players go through the same athletic training. It's easy to close that gap. The new players realize how to properly train for the game and everyone becomes bigger, faster, and stronger. But the talent gap still exits. It is much harder to develop that understanding - the technical/tactical developments. And the group the initially dominating lags behind.
At the onset of women's football, American girls were better athletes. They had played basketball and softball and other sports growing up and professionally. They had the advantage and it is evident by the history of the sport. Now, the talent gap is closing. Teams like Japan, Brazil, Germany, and France have some of the most technical players in the world. And while the Americans still are at the top of the sport, I expect it not to last much longer. As women's football grows more and more popular, and more and more girls throughout the world pursue it as a career and a passion, the underdogs will close in on and overtake the Americans. The USWNT loses girls to other sports and relies to heavily on athleticism. The same reason we were initially the most powerful team in the world is leading to our downfall.
On the men's side, it is perhaps a different story because the game has a much longer history than the women's. The men are at the stage where technical ability is at the forefront of success (see Barcelona). The German national team is the defending World Cup champions and certainly deserve to be the number one team in the world. Their players are the most technical and proficient. Their tactics are beautifully choreographic. But they don't have any 6'7" athletic freaks. They have footballers. That is what is required to win matches. Men's football is beyond the stage of "bigger, faster, stronger." The women's game is still in its infancy, but I believe it will grow out of that stage. And that means trouble for the Americans. I hope you celebrated quite a bit at the recent American victory, it may be the last one we see in a long time.
On an unrelated note, my mom and sister came to Leipzig a few weeks ago. We also saw Dresden and Berlin together. I must thank Hobart College not only for the opportunity for me to experience something remarkable, but also giving my family the experience as well.
