"Balance is the key to success in all things. Do not neglect your mind, body, or spirit. Invest time and energy in all of them equally - it will be the best investment you ever make, not just for your life but for whatever is to follow."
-- Tanya Wheway
The human body requires balance to function. In training, that means not neglecting any muscles, in regards to their strength and flexibility. You can have all the quad strength in the world, but you won't be able to utilize it in a high speed movement like a jump unless your hamstrings are both strong and flexible enough to control forceful knee extension. You must have balance between these two dominant muscles in your leg. In the same way, your hip flexors must be able to thrust the hips forward to create the propulsion for the jump, yet many people neglect the hip flexors in their workouts. Sometimes, people also fail to develop their ankle strength and mobility, causing them to lose range of motion in their squat as well as put them at increased risk of injury.
I could go on for days, listing the parts of the body that are seldom considered but affect each and every movement we make. The bottom line is, you need to be strong and flexible everywhere to maximize your jumping ability.