I would say for the same reason why Satoshi designed Bitcoin the way he did. To create a centralised protocol that is censorship resilient. One is the money, the other is the speech and digital media.

An easy approach to exposing the crawlers to nostr and nostr apps would be for apps (in particular web/browser apps) to add static pages to their sites for the mere purpose of SEO. About, Facts, nostr and app relevant faq, terms, etc etc. Those pages will be indexed.