Full article here.

TL;DR:

in order to ensure safe and adequate blood supplies and equitable access to safe blood and blood products.

• Safe: If you are volunteering, as opposed to being compelled to donate as a family member or being paid for it, you’re less likely to conceal things about your health that would make your blood dangerous to donate.
• Adequate: Countries with 100% voluntary blood donation have more people donating blood regularly, meaning a bigger pool of blood you can expect to have of any type.
• Equitable access: If you just compel friends and family to donate, what about people who don’t have them, or whose blood is incompatible with that of their family and friends? If people get paid for it, watch as the poor people all give to get some money and watch as it devolves from “help others and get paid” to nasty commercial exploitation of peoples’ bodies. And that payment of donors has to come from somewhere, increasing costs of treatments that need transfusions.

There are more reasons I didn’t list that are listed in the article :)