i got your gift
Donors want to know that you actually received the funds that are sent, and the quicker that they can get that information, the better. It is easy to receive a gift and in the busyness of ministry (or life in general) to say to yourself, "I will thank them tomorrow."
And, we know that tomorrow often doesn't come for several weeks or several months as other, apparently more urgent issues push saying "Thank you" to the bottom of our agenda.