Rather than go over it again, here's my speculative answer to why BS has grown so much quicker than Mastadon - which isn't quite the question in the OP, but will cover much if the same ground (That whole thread is worth a read IMO.)

TL:DR - It's different because it isn't shit. Yet.

And it might not become shit. I think it's a shame that people are so instantly cynical about the possibility that things might turn out alright , especially if the people using it go into it having learned some lessons from Twitter, and with a determination not to repeat the mistakes. At the moment BlueSky is fun and friendly, and I'm going to enjoy it that way for as long as it remains the case. If it goes to crap, then I'll move on.