I know someone who recently told her SKS the reason and it pissed both BPS off.
I personally have been asked this by my own SD but direct her to ask her mom and dad. Her mom has told her it's because they were to good of friends to be married. The actual reason was because her mom had sex issues (not naming them) and would for months not have sex, and her dad repeatedly cheated on her.
Neither of them want their daughter to think bad of the other parent. Why would someone else think it's okay to answer this question for the ones it actually happened to? It should be the people that were a part of that relationship to tell the kids why it ended. IMO