Because charging for readings would associate you, in people's minds, with every other "psychic", who is only looking to make an easy buck by fooling the weak minded and/or people desperately looking for answers (a type of "psychic" that is saturating popular culture, as well as 1-900 hotlines). If you consider yourself a true spiritual medium, would you feel good about making people think that those charlatans are more credible, due to you practicing the same profession? I think there's a reason that people who really believe they have a gift, and use it to help people, don't make a paying job out of it. The above, plus probably a sense of self pride, and ethics.
But what do I know? I have trouble believing even those who truly believe themselves that they have some kind of spiritual gift, actually have something going on there.