Podcasting was built on RSS. At the time JSON did exist but nobody had yet used it as an interchange standard. It was just a way to serialize js objects. XML was what all serious tech used. Only later did we get json schema and representation of feed like content in json.

By the time activitystreams came out we’d switched to JSON which is also used by activitypub.

Lots of publishers, including podcasters like NPR do publish in using jsonfeed.org but I’m not sure which podcast clients support it.