We're talking about a vaccine here, not a cure.

This is preventative medecine, as far as I know, most vaccines cannot be given to someone who already has the disease.

So it makes sense to target healthy people first, to test out the vaccine side effects, make sure there aren't many or bad ones, and then test out if the vaccine works. And then once you know it has low side effects and works, you can risk testing it on more compromised individuals.