Also, manufacturers can make stepped blocks more easily than bowed. Also, I can venture to guess that stepped might be more beneficial to quads due to the larger pressure area compared to the bowed one, which have more pressure on the center. A flat block will be limited by the uneven IHS because in most cases, the corners will be slighty higher than the center so the contact is poor. Stepped will avoid the corners.

Ease of manufacturing, ease of lapping if needed (not possible with bowed), potentially better with quads and larger contact area.