The difference between the incidental givers, people who respond to a knock at the door and hand over a twenty-dollar bill, and the people who put $50, let's say, on the offering plate at their place of worship, is huge. The median annual donation for people who are incidental givers is somewhere barely above $100 a year. It's approaching $1,000 a year for the second.
The second kind all know about tax credits. The first kind, who are somewhere in the vicinity of two-thirds of donors but who account for a minuscule portion of all dollars donated—they don't know about it. So it becomes an issue of how to change the first into the second.