Question

why should seriously ill people have the priority to receive donated organs ?

Answer 1

Because their condition if more critical, they're more likely to die sooner than someone with say, more time left, so receiving that organ would seriously save their life.

Answer 2

Because if they really need it, it should be given to them so they can live longer. It would save their life, and the person donating the organ is usually healthy, helping even more to the person in need of the organ.