Married households fare well because the tax system and plenty of other institutions favor married households. It doesn't have to the the case.

Marriage is mostly a "capital" redistribution system and with strong public services (ie shared services) there is much less need to focus on the nuclear family as a protection (sharing of ressources) network.

Ex. When a few years back pension laws changed in Japan and housewives got the right to get a pension separately from their husband, the rate of divorce boomed.

There is nothing in the nuclear family that makes in a natural fit to what humans are as social animals.