Hunger. People are already living much longer than the Earth can sustain. Curing cancer would mean that more people would live to be older which would help accelerate population growth and further our trip across the line of Earths carrying capacity for humans. Curing hunger on the other hand doesn't explicitly keep more people alive longer. At minimum, it provides them with a better quality of life. Ending world hunger also wouldn't result in as many people living longer such as curing cancer would.