Why do people believe they can send out one mailing, one
time, and all of a sudden become bombarded with new
customers?

It's ridiculous to believe any one shot mailing is going to
uncover everybody interested in your service or product.

Think about your own life. You've probably received some
offers that you were interested in but you never acted on.
Perhaps you went on holiday, had to prepare a product launch 

or a number of other reasons. Your prospects are the same way.

To overcome this, you've got to create a plan for multiple
contacts. Also, this way you can take advantage of the
"moving parade of life". You are not selling to a standing
army, but rather a moving parade of prospects. People will
move in and out of different needs and wants as they go
through changes in their life.

Here's an example: Let's say you're an owner of a electrical
shop and you sent me an offer for deeply discounted
appliances. Well, you probably wouldn't have much luck
because my house is full of appliances. But what if when
your second (or third or fourth) offer came, my fiancée had
left me and taken with her all our electrical appliances. 

Now, all of a sudden, I'm a *very* eager customer.

That's why using multiple mailings often leads to

multipliable  returns.