Because the glass elements in a camera lens are round, lenses project a circular image onto a camera’s sensor plane. This projected image circle must be large enough to cover the rectangular sensor, like so:
Lenses designed for Nikon DX generally project a smaller image circle because they only need to cover the smaller DX sensor. This enables a DX lens to be smaller and lighter, but also means that these lenses are not suitable, by design, for FX cameras. For the Canon ecosystem this law is absolute, as EF-S lenses, designed for a smaller APS-C size sensor, cannot be mounted on full frame EF bodies.