The word “Pledge” has a negative connotation. For many, they won’t even consider an extended payment plan.
You surely have heard this sort of a comment: “I don’t pledge (making the fatal word three syllables!). I’ll make a gift, but I don’t know how things will be in the next few years. So I’m not going to pledge.” Or, “I just don’t believe in pledging. I’m almost certain I’ll give again next year, but I won’t pledge.”