They are both emerging networks with a different user base.

As the Nostr quickly gained popularity amongst Bitcoiners, there are larger percentage of users here who share common interests.

Due to the changes made by Elon, the consensus on Bluesky is that Twitter is now a right-wing platform. Accordingly, it is being more heavily adopted by those on the left.

I suspect this will change over time, but not until both protocols gain greater adoption.