Because you might slip and injure yourself, and the tendency when people jump is to move upwards...with the giant stride entry you want to move outward laterally....you step forward with an extended stride like motion so your body and tank clears what you are standing on, bringing your legs together once you hit the water assists in returning to the surface.
This entry is derived from water-safety/life-guard/rescue entries where the person doing the rescuing needs to enter the water without taking their eyes off the victim. Performed properly, the giant stride entry allows the rescuer to enter the water without the head submerging. If one were to jump, the tendency is the increase upwards movement results in increased downward force/movement causing the head to submerge possibly resulting in the rescuer losing track of the victim.
-Z