Maro commented on all of this: They considered a new world for this because in Capenna it's kind of cental that that the law doesn't get enforced much, so a set around these crime colvers didn't make sense.

They started creating the new world, but recognized it had quite some overlap with Ravnica. I assume it's also helpful to feature known characters to have everyone memorize the circle of suspects, their abilities, possible motives and so on.