People don't respond because they may be constantly getting hit up for $. For example, DH is director of a division of 210 people or so across the world. They are a pretty fit bunch; if he gave to just one of them, he would have to give to all. He gets at least one e mail a day for money from people he works with. So, he doesn't give to anyone.
This is why I don't do charity rides, except for the kind that just have a registration fee with no obligation to raise more. The only one I did with huge fund raising commitments was paid for by DH's employer. I just can't ask people for money. I *do* give a contribution to the few people that have asked me over the years... maybe 3?
The whole thing about raising money to ride 50 or 100 miles leaves me uncomfortable. I am not sure why, as I do choose to give to various charities on my own.