Marriage is a huge commitment, and couples understand they are joining their life and soul with another person. Some people are unprepared to make a commitment this big. It frightens them for any number of reasons, and it has nothing to do with the person they want to marry. Rather than work through the issue, many simply try to avoid it. In the end, they may decide not to show up for the wedding.
Leaving someone waiting at the altar is the height of humiliation. Many weddings include family and friends of both partners. Jilting someone in this manner leaves them to be humiliated in front of people they know and love as well as people they may not even know. It is a painful way to find out someone does not want to commit to marriage. If commitment is the problem, finding the courage to tell the other person before the ceremony is best for everyone.