The tl;dr is that people donate a lot more when they can tangibly understand that a disaster has happened and they make an emotional connection to the disaster (e.g. 9/11, Haitian Earthquake, etc).
When donors are asked to contribute money to prevent something, the emotional connection is a lot harder to make and funds are a lot harder to raise.
http://www.npr.org/blogs/money/2014/09/26/351515481/episode-...
The tl;dr is that people donate a lot more when they can tangibly understand that a disaster has happened and they make an emotional connection to the disaster (e.g. 9/11, Haitian Earthquake, etc).
When donors are asked to contribute money to prevent something, the emotional connection is a lot harder to make and funds are a lot harder to raise.