Because they don't want your blood if you have a higher risk of diseases. You have to fill in a questionnaire with all kind of questions relating to your health, travels to other countries, your sex life, dug use and a few other things.
The idea is that you might not be completely honest if you get paid for your blood.
Have you done some coke last month? Just tell and nothing will happen, except you won't be donating any blood for the next 11 months.
Have you done some coke last month, but also really need money to feed your kids? It might be really tempting to lie and get that blood bank money anyway.