Well, first there are strands of magnetic field left inside, but now the superconductor doesn't like them moving around, because their movements dissipate energy, which breaks the superconductivity state.
Pa, imamo silnice magnetnog polja koje su ostale unutra, ali sada supravodiču više ne odgovara da se one kreću naokolo jer njihovo kretanje rasipa energiju i to prekida stanje supravodljivosti.