Designing a lens for crop implies that it has been designed for a smaller size sensor and therefore it won't deliver the same image quality outside of the frame of a crop sensor. All the glass in this lens will be optimized only for crop sensor sizes. Possibly image corners may be blocked or they are at least otherwise affected.
Although I am unsure, if this applies to this particular lens, it is also possible that the rear lens element of a lens may protrude too far into the camera body, so that it won't work on a full-frame body, which would have a larger mirror compared to a crop camera. This would be the worst case scenario, but I don't know, if this applies to this lens or not.