Porque en todos muere mayu ;_;

No, no en todos muere Mayu. Hay un final, donde todos excepto unos vuelven al pasado condenados a repetir los mismos hechos. Es uno de los finales malos, pués "todo vuelve a empezar".

Si no estoy mal informado, el juego sigue ese final...

"Corpse Party: Book of Shadows will add new twists and new backgrounds for characters which were barely touched upon in Blood Covered (such as Mayu), plus a few details that will be important for eventual sequels. For the most part, the game actually plays as a prequel outside of the odd chapters which appear to continue from wrong ending 6, when Sachiko sends the protagonists back in time, erasing their memories except for Satoshi. He tries, in vain, to dissuade his classmates from performing the magic ritual, but he joins them since he doesn't want to let them go alone. The game is divided into seven chapters which are not connected to each other:"
http://corpseparty.wikia.com/wiki/Corpse...of_Shadows

Ya veo, la verdad es que tiene pinta de que no se esperaban el exito de la primera parte y buscan abrir las puertas a futuras secuelas, en cierto modo no me gusta porque me gusto como acababa la historia original pero puede ser interesante lo que salga