In normal space, the speed limiter is present because past a certain point the ship's shields and hull are no longer effective at blocking cosmic particles. At higher speeds than the ship is rated for in normal space, you'd likely die of radiation poisoning or from being pierced by microscopic particles.
In supercruise and hyperspace, the Alcubierre drive is effectively bending space and time so the same physical issues no longer apply.
In supercruise and hyperspace, the Alcubierre drive is effectively bending space and time so the same physical issues no longer apply.