Because if that roller coaster stops unexpectedly for any reason, its lap bar won't be able to restrain a child under 40 inches tall. That's why the ride has that height restriction. Ignoring ride restrictions can kill riders and has on thrill rides around the world.

Height requirements for roller coasters are primarily based on the physics of the restraint system and the safety of the rider during high-G maneuvers. Restraints (like lap bars or over-the-shoulder harnesses) are designed to fit a specific "envelope" of human body sizes. If a rider is too short, their shoulders or waist may not be broad enough to be securely pinned by the harness, creating a risk of the rider sliding out during inversions or sharp drops. Furthermore, height serves as a proxy for physical development; shorter children may not have the neck muscle strength or bone density to safely withstand the intense centrifugal forces and "snap" movements of a high-intensity coaster. There are also mechanical reasons: sensors on some rides are positioned to detect riders above a certain height to ensure they are properly seated. These limits are strictly enforced by law and park policy, as even a few inches of difference can be the factor that prevents a rider from being properly "locked" into the safety envelope.