Understand that we're discussing a coach for a National Team here. That's different from a club team.
For a club team, you have contracts and farm systems (academies) that mean you can go after players that fit "your system". You can have a preferred type of soccer tactic and get players that complement that system. Those players are together for most of the year, working out and practicing to do what you and your assistants tell them. As a manager, you're picking the starting 11 players each game, and making tactical substitutions based on how the game is going.
For a national team, you only see your players at certain times throughout the year, and over the course of a four year World Cup cycle as it goes. The players that are available to you are ones that are in the national player registry - legally able to play for your country. During one of the windows of international play, you call in a certain segment of those players to participate in a camp. At the end of that camp, you pick your roster (usually limited to like 25 players) and go set up for a tournament, qualifier, or what-have-you.
The kicker is how you select your players. Maybe this guy's usually a really good player, but he hasn't had any action with his club team. He's a substitute or little used. You're asking him to jump right in and attack the game with no pre-season or other type of form. Maybe another guy looks promising, but he's been playing for a weak team in a weak league in eastern Greece or something. Another guy is recovering from injury. Still others have no experience on the national stage, but could be useful later? Do you develop them?
Of course, the ultimate prize is playing in the World Cup tournament every four years. To get there, you have to qualify by winning games against other teams in your region. The USA is in the CONCACAF region over here with Canada, Mexico, and the Central American nations. Mexico used to make some noise in the tournament, but really, no teams from CONCACAF really strike fear in the rest of the world.
Getting a manager that can use the USA's European developed (we hope) players to quickly jell, form a tactically sound (there's zero time to teach or develop skills on the nation team), cohesive squad that can adapt to the defensively aggressive styles (read: they foul hard and often) played in our region will require a world view of soccer acumen.