Bitcoin nodes use proof of work to ensure that our nodes can all agree on the current state of the Blockchain. However, I can't afford to shit post on those nodes because the consensus mechanism forces all nodes to store the same thing - which doesn't scale to many posts. 

Nostr relays (and git remotes) don't have such a consensus requirements, which means that different relays may give you different profiles pictures. This seems annoying, but I also don't want all relays to agree which of my content to store, because in most cases they would agree that it's too costly to store any of my content.