I'd imagine the reasons why some relationships without commitment last longer than others differs quite a bit. But possible reasons might be it becomes comfortable, someone doesn't want to be alone, codependency issues, etc... Perhaps those are ones that might of made it if they had made a legal commitment to the relationship? Although there's another statistic I've heard of that people who live together and then get married have a higher divorce rate. I think society's acceptance of divorce and people living together has made it easier for people to leave situations they don't find to their liking.
Another thing, this will likely increase... the other day there was an article in the LA Times about the younger generation (I think Gen Y?)... recent college grads being the most narcissistic generation to date. Narcissisim is a trait that makes close, long-term relationships difficult to maintain.
There is no arriving, ever. It is all a continual becoming.
