You move to another state. Every state has different laws on wills. Your executor or executrix can't serve. For reasons such as death, illness, moving away or out of the country, or perhaps losing your trust. Your marital status changes.
Spouses pass away and other changes occur, too.
You have more children. Many late babies are accidentally disinherited, due to certain state laws and forgetting to check an old will. Your guardianship wants to change. Your children grow up. They become independent, get a job and marry.