How Do You Explain a Deathbed Wedding?

The reason is that if the couple have been previously married invalidly, then they can receive the grace of the sacrament before the death of one of them, and that is no small benefit to their souls. Even if they were not previously married at all, the bestowal of the grace of the sacrament is a more than sufficient reason. Of course, this is not an obligation. Sometimes also there can be economic reasons of inheritance and so on that might indicate that they should be legally married to protect the interests of the surviving partner.