Why is Elvis Presley living in a convalescent home in East Texas? Because 30 years ago, he escaped the fortress of his fame by trading places with an Elvis impersonator. Why is Elvis doing battle with a mummy who haunts the rest home so that he can suck out the souls of the dying? Because Don Coscarelli, writer-director of the logy, fatuous Bubba Ho-Tep, is trying to will a cult movie into existence -- which, of course, never works. It's a small plus that Bruce Campbell, from the ''Evil Dead'' films, refuses to play Elvis as a joke, but he's stuck chewing over a litany of old-hat regret (he should have dumped Colonel Parker, etc.). Besides, how much fascination can the King hold when he's attempting to smash a giant scarab with a bedpan?