In the United States, at least, you would find many people who would argue that the premise behind this question is false. Many people no longer believe that a husband is greater/more important than a wife. If we assume that this is true, though, I would point out three reasons that have traditionally made husbands "greater."
- Religion. Many religious traditions hold that men are superior to women. The Christian Bible, for example, tells wives to obey their husbands. This sort of thing has made men feel superior to women.
- Physical strength. Men are typically bigger and stronger than their wives. These attributes have allowed them (especially in the past) to be physically controlling. They also help make people feel that men are in some way better than women because they are stronger and can do more things, physically speaking.
- Economics. Men have traditionally been more important to a family in economic terms. For reasons of strength (and because they're never pregnant), but also for cultural reasons, they have often been the ones who contribute more to a family in economic terms.
For these reasons, husbands have been seen as more important and "greater." However, you will find these attitudes declining in many countries today.