Those who supply the annex with their goods and services, why do they do it?

Why? Because they love the family and they're good people. You have to remember that if caught these people would have been shot; everything they did for the frank's would have warranted execution, but their love and humanity won out over fear. These very courageous people did the right thing out of love, and I don't doubt that if needed they would have done the same for anyone if it was in their power to do so.