>>280231588His assistants didn't wanna become jobless, and his old friend thirsted for credit as "co-author of Berserk", since his own mangas are mediocre slop at best. Also Berserk is the cash cow of Young Animal, so I'm sure Hakusensha didn't mind either.
Basically a perfect storm of all the wrong reasons to continue the manga despite the author dying. But it's all to honor his legacy, of course.