well maybe they were trying to make it work, maybe everything else was great, but there was one thing holding them back from making the lifetime committment
What kind of insecurity? when they both love each other and other than that he/she is marrying to some stranger so how can that stranger assure security? Well i guess you are pointing to "marriage" as a security thing here.
I guess it's pressure on every side including the family and between the couple as well. Still it's something so personal that only the individual making the choices knows the truth, we can't really comment on that. Sometimes it's family pressure, sometimes they think the made commitment with the wrong person...sometimes they just get bored with their partner and sometimes they don't have enough courage to stand up for themselves. There can be hundreds of reasons. But one should think about them before committing to someone as well.