User blog comment:B214/Question and Answers 6/@comment-73.57.25.92-20180712215024/@comment-28884330-20180712234656

Because Ishibumi became more edgy about him, perhaps. Jk.

He begins using his scythe in Volume 2 from one of the illustrations, so it will be revealed when that's translated. The Sacred Gear does still use swords, though. They're just not used for the close combat component (except for Jin and the other dogs. THey still use swords).