Paying only for results is the holy grail of business. After all, you're not in it for a Participation Trophy, right? So why should your vendors?
Your definition of results and theirs are probably not the same. For example, you might want to pay on revenues. But they'll say that, since they have no control over what happens after they generate the lead, they can't do that. So you compromise, and agree to pay on Lead Quality. And then the arguing begins.