First, they require equipment and labor to set up and maintain. Second, a minority of individuals who use remailers are a pain in the neck. These selfish persons drive remailer operators into early retirement. Third, many remailers produce zero revenue. Several years ago, there was no technical solution to these problems.
Privacy is valuable.
For example, offshore banking is one of the world's biggest businesses. In order for remailers to become commercial on a big scale, anonymous payment systems such as DigiCash must become popular. My predictions came true. Today, many remailer operators charge fees for the same reason that you go to work in order to pay for food, housing, etc. Some people set up remailers for their own personal usage, which they may or may not care to share with the rest of us.