A number of reasons:

• For a number of people, they just could not fathom buying sex. Part of that is some kind of pseudoreligious idea, part of that is also that they just don't understand why you would want casual sex. These people often don't understand why in the hell you would want to be a prostitute or a John, and don't see why it shouldn't be illegal.

• Another number of people see it as "losing". As in, you would only ever want to buy sex if you've failed to get it some other way. Essentially, if you want to buy sex, something is wrong with you. So they look down on those who buy sex (even though they're sometimes secretly those who also buy sex themselves - sort of like the anti-gay republicans who get caught... well, you know).

• There's also the view that no woman in her right mind would ever want to sell sex, framing selling sex as abuse. Never mind that that's not necessarily the case and that selling sex is not just something women do, but if you see it like that, it makes sense to outlaw it, since you're trying to remove a source of harm.