There tend to be a few reasons that pop up more frequently than others.
Changes to Financial Situation
People hit retirement, a spouse stays home with kids, having kids, aging parent in need of help, or some other event has a large impact on budget. When people review their finances, they make changes. Many times it impacts charitable giving.
A lot of charities are geographically bound. It might be a shelter for the metro area, or a outreach to an immigrant community in the city. Having proximity to the nonprofit creates a sense of community and helps drive the need to get involved. If I run an after school literacy program in Minneapolis that you support, but then you move to Miami it would make sense for you to stop donating.
Changes in Seasons of Life
When I was a kid I was involved in cub scouts and later boy scouts. I loved camping and learning about the outdoors. My parents donated to the organization in addition to any membership dues and fees. When my brothers and I left so did their support.
When we invite people to join us in our nonprofits we are inviting them into an area of major passion for us. With strong passion comes strong opinions that can lead to a stubbornness around how things ought to be. Conflict is going to happen, and when we don’t expect it, or respond to it poorly, we loose people who shared our passion. Life with people is a messy endeavor, but we must work together on the hard problems before us today.
Major Life Events
It could be a death, a birth, a major accident, or some other event that causes people to assess life and make changes. Many people who loose loved ones to cancer get involved with nonprofits that help with research & support. Having a friend move to an abuse shelter could spark a new passion. Children are major life events that are so full of unknowns that could lead to changes in donations.
But, the biggest reason people stop supporting a charity…
They stopped engaging with your mission
Ask someone why they stopped giving and you’ll hear any of the reasons above. Yet we know exceptions to each of those. Each of those exceptions will have one thing in common – they are in love with your mission, and how you are doing it. They are likely so in love with it that they volunteer, participate, and promote your nonprofit.
The obvious follow up to your question would be how to keep donors around. The answer is to keep them involved, informed, and to tell good stories of your work.