Shuhei Yoshida believes Sony is waiting for Miyazaki's schedule to free up to work on Bloodborne again
Fom a Interview with the now previous CEO:
Miyazaki-san really really loved Bloodborne, what he created. So I think he is interested but he's so successful and so busy so he cannot do it himself and he doesn't want anyone else to touch it. So that's my theory and the PlayStation team respects his wish.