"They didn't get along" hides a lot of complexity, since a lot of concrete things can put strain on a relationship and then the couple "doesn't get along". In fact, this practically reduces to a tautology, since deciding to get a divorce means that you're goiong through something severe enough that staying together isn't considered to be the better option.

Infidelity is more often a symptom than the actual root of the problem. While some people just can't help themselves, more often what happens is one or both aren't getting the support they need from their spouse and they start looking elsewhere because of it. Then cheating, fighting, divorce. Not really an excuse, so much as an observation.