Why do the Nationals have such a good record? Why do the Royals have such a bad record? There's no single explanation for anything as complicated as this, but I can give you a hint as to what has been most significant -- the Nationals already have outscored the Royals by more than 100 runs. The Nationals have scored half a run a game more than any other team. The Royals have scored half a run a game less than any other team. In terms of just run prevention, the Royals have been better than the Nationals. I guess you could say they've been defined by run prevention on both sides of the game.
It's the easiest thing in the world to identify who has had the best and worst offenses. It's more challenging to think about who has the best and worst offenses. That requires an examination of the future, which, of course, can sometimes feel entirely unknowable. What's going to follow here is my own picture of the baseball landscape. My own leaguewide ranking of the MLB lineups.
Because of the format of a post like this, it can look like very little thought has gone into it. but nothing in here is arbitrary. So far, the Nationals have been the best, and the Royals have been the worst. I'll tell you here I don't think that reflects their exact abilities, but that's why we have the rankings.