The first two images to the charts aren't working. They show up as dead images here.
Otherwise, there is a simple explanation on why American Splatfests are like that and the Japanese and Europeans ones are not: Children. Or rather, the way children play video games is different in American than in Japan or Europe. Japanese culture stresses to give it your all in everything you do, so even kids will play very well. Many European countries have a competitive culture (I can think of the Netherlands, Spain, Italy, Norway, and the UK, and to a lesser extent, France, Germany, Russia, Denmark, and Sweden), where everyone is encouraged to win but to also take losses in stride. Also, due to the sensitivity much of Europe has about violent content, video games are more associated with teenagers and adults in Europe than children, so there are fewer children playing Splatoon than in other regions, proportionally.
In America, however, video games, when played by children, are seen as a way to pass time and to keep kids occupied while their parents go do something else. In addition, kids start playing video games at a younger age than the other regions, spurred on by mobile gaming's simple controls and low prices. Because kids are pretty much left to themselves to do as they please, you get a lot of little kids playing who might not have even developed motor skills and are not really aware they're playing against other people. There's also the predominant means of parenting in the United States and Canada, which is to cheer them on and say "good job" in everything they do--while this does have its benefits, it also means they don't really know what failure is, and there's little reason for them to try very hard at anything.
