The twitter refugees that went to mastodon and gave up likely never even tried soapbox or the good mobile apps. Search Engine Optimization for the fedi has been fucked since the media blitz of 2017. Also the walled garden instances joinmastodon.com funneled them into probably didnt tell them about soapbox on purpose.

Also, a lot of the people here came from twitter and in many ways nostr is more similar to it than the fedi is. We can't really compare instances to relays because there's no real loyalty to them here. People publish to as many as they know about and (hopefully) pull down posts from just a few. Whereas on fedi people identify with their instance and the instances close to them and communities form. Communities on Nostr are more like twitter. Just like minded posters in a giant public pool with no clear boundary. I'm not sure nostr will ever replicate what the fedi has, and I think thats ok