Businesses want customers. Granted they get the chance to get their brand in consumers minds, but they would much rather get the brand established by making a sale. Contrast this with affiliate programs.
Most affiliate programs pay for actual sales.
Meaning that you, the affiliate, only get paid, when someone you refer, buys something or signs up for a service. Paying affiliates for each sale, guarantees that the businesses pay the least amount, to get the most customers. Affiliates want revenue. You have taken the time to create a site, and you are starting to receive some visitors. You might have created the site for the pure joy of it, and you just need to cover some of the expenses, or you might have created the site with the idea that you wanted it to get lots of visitors, so you could make some money with it, and now you want your chance.