It is much bigger.  10x-100x 

 Better user experience? 

 A bit.  But they invested in infrastructure and it paid off big.  In truth, nostr would not be able to handle a faction of bluesky even with a perfect UX. 

 A fraction of the traffic? 

 Because Bluesky is centralized 

 It simply got a lot of MSM coverage at the time Elon bought Twitter. The herd followed. And yes, likely because of a bigger audience. 

 I think it just comes down to simple norm user onboard, and it looks a lot like twitter did, and when they get there the users are more diverse.
We need nostr clients onboarding on interest rather than just recommending bitcoin/nostr folks