Technical reasons, it was to stop the player going into the towns so there was more information for the spectacular end FMV. The developers did the same by blocking off some towns in FFIX and removing the information from the disc so there was more room on the disc for the ending. However, the 'blocking off' on the final disc of FFIX was much more cleverly done than the clumsy, unexplained way it was done in FFVIII.