Because you can't upload a podcast's actual audio files directly to Nostr relays, so it's much easier for everyone (creators, clients, servers) to host the RSS feed with the domain hosting the content in the first place. There's no reason why you couldn't *also* have a NIP for additionally publishing and consuming feed updates on Nostr relays. But the protocol is working very well as it is, and is used by hundreds of millions of people without dramatic censorship happening. Which, yes, is one of the important reasons for defending it.

That said, RSS is extensible, so you could easily add new attribute types to feed items to e.g. point client apps to social interactions on Nostr.