Vaccines are hard to develop, and can be dangerous. This is a vaccine for a disease where 40% (at current estimate, up from 35% before) have no symptoms, and where only about .4% die. It’s also a widespread disease — this isn’t Ebola where you could vaccinate 50,000 people and bring an outbreak to an end. You’d have to vaccinate a billion or more. When you vaccinate huge numbers of people for a disease that isn’t very dangerous, your vaccine has to be very safe to be better than the disease. That doesn’t go well with a rushed development effort.
UPDATE: Related: Merck CEO says raising COVID-19 vaccine hopes ‘a grave disservice’ – report. “If you’re going to use a vaccine on billions of people, you better know what that vaccine does.”