Not to be conspiratorial, but I kinda suspect politics at play here... With most of Nostr reflecting a "right-leaning" culture of Bitcoin, while the Fediverse being pretty heavily left-influenced.
I also don't necessarily want to absolve the Fediverse, in that many instances don't federate with the bridges... But plenty of instances do Federate, yet when I reply to posts, I don't see my comment show up when I go to said instance outside of Nostr.