Can’t tell you why it is, but couples contemplating divorce, particularly when they’re jockeying for position, often agonize over which of them should file. Warily, they circle each other like punch-drunk boxers, waiting for the other party to take the next swing so they can spot an opening and exploit it.
I’m not sure it matters as much as people sometimes think. The culture sometimes (naively) assumes that the person who filed divorce is the one who wanted the divorce, but anyone who deals with divorce regularly knows that’s not the case. The plaintiff is just as likely to be the left as the leaver.
In litigation terms, the plaintiff in a divorce case presents his or her case first. Other than that, I’m not sure there’s any advantage or disadvantage (once a case is underway) in being the plaintiff or the defendant.
HOWEVER, there may be an advantage to being the plaintiff if it allows you to choose the forum. For example, let’s say that you and your spouse both have minimum contacts in two states and that one of the states is more liberal with alimony than is the other. If alimony is an issue, there may be an advantage to being the first to file, so you can choose the state that’s more favorable to your position.