we're past the end of the year rush from people showing family and friends at holiday social functions. and in the phase of hectic season (taxes, crunch quarter/semester for school). combine that with things like meta dragging attention to their fediverse stuff.
but beyond externalities, internally nostr still suffers from an echo chamber effect and an increase in relative toxicity as compared to twitter. A good number of those active pubkeys are likely also bots. real regular userbase is probably just a few thousand