As others have said, there can be many many reasons.
Too many people though, want the fairytale wedding and plough all their time and effort and money into organising that ONE DAY, without having considered what happens AFTER the big day. They have not talked at all about their desires for a family, how they will sort their finances, housework, where they will live, having independent spending money/hobbies/social life, regular commitments to visiting ILs etc etc
if both the partners have had their own set ideas of what it will be like, but have not discussed it, or even worse, have assumed they can get the other person to change their minds after the wedding, then you have a recipe for a disaster.
I actually think that the longer you spend planning a wedding and the more you spend on that one day, the MORE likely you are to split up TBH. Of course there are exceptions to the rule, but for so so many young couples the "dream" is too focussed on the day, rather than the happy ever after, and that a relationship needs both people to work on it.