I think the easiest explanation is that they no longer enjoy the presence of each other, at least romantically (some divorcees remain good friends), and that the improvement in their life quality is worth the effort and fallout (if any) of a divorce.
To get deeper, there's the expectations they have of each other (if they've been voiced; it can be hard if they don't even know what they want or how to communicate them), which if are sufficiently breached (cheating, not pulling their weight at home or financially, etc) communicates a lack of respect for the other that once sufficiently obvious results in bad feels between the two (one angrily harping on the other, the other not caring, etc).
Marriages can work if the boundaries are set at the beginning, and are well-understood, but it can be difficult to really work it out unless you've had enough life experience, but many first marriages at least don't. Married people can operate independently of each other without expectations of the other, but that's rare.
If they have children, that exposing the children to the relationship that the parents have with each other would be unhealthy for the children.
Or, they had no idea what they were getting into and/or are mismatched, and decide they didn't want marriage after the fact. Or one extremely changes after the marriage. Many more reasons too.