I think that perhaps those who have experienced it have a good insight into it – from their personal perspective, but that is certainly of more significant value than rank speculation.
Though my experience was different than Health’s experience, and maybe we drew different conclusions, I respect his perspective on this, as he has the benefit of experience, as, unfortunately, I do. (Though I can’t say completely unfortunately. I have wonderful kids from my first marriage, and without the experience and personal growth I undertook after my divorce, I wouldn’t have met or had the tools, humility, and commitment that I brought to my relationship with my new Kallah.)
I don’t agree that people dafka give up because its now more socially acceptable. Plenty of people try for years to make it work, seek guidance from their Rav, seek therapy, and make positive change in their lives. It doesn’t always work.
To answer the OP, In my experience and that of peers who have also gone through divorce, the key element that was critical that led to almost all other problems was communication. Was it respectful or antagonistic? Was it constructive or destructive? Were all issues aired or was one party not communicating important or critical matters to the other with honesty and clarity? Was one dictating and expecting immediate acceptance? Was the other holding in things that ought to have been aired rather than fester? Were responsibilities clearly communicated to each other? Were both as parents on the same page and a united front when it came to dealing with the kids? Did they have each other’s backs when they were among others?
A secondary element related to communication is expectation. If expectations aren’t clearly communicated at the outset, someone will become disappointed and disillusioned very quickly. And even more important, if one spouse has an expectation that through their efforts the other will change without the other undertaking that responsibility for change for themselves, they will be disappointed.