Asklemmy

Having used both, here my view on why BlueSky is outstripping Mastadon:

• It is instantly familiar in operation to anyone who has used Twitter. It looks and feels almost the same to use in a way that Mastadon doesn't (arguable whether that's a good thing or not, but it makes for a comfortable transition).
• There's no messing around with instances to negotiate - you go to bsky.app ~~BlueSky.com~~ and it just works. Hard to overstate how important that is in retaining people who take a look at a new platform.
• There are a lot of people on it, it doesn't feel empty like I have often found Mastadon.
• There are a lot of relatively influential people on it, media people, authors and actors and comedians, who have largely shifted as a single mass (probably due to the three above reasons) - so for non-famous people there's a sense of being in touch with what's happening.
• It's riding a wave of positivity about itself, which Mastadon never had - this touches on your point about media coverage of it, but whether that's really due to money being paid to news orgs or just due to journalists seeing what they are doing as being important for others to know about is open to question.

I think the various high profile organisational defections to BS have been a big part of it too. I only looked at BS for the first time when I saw the story about the Guardian newspaper quitting Twitter.

I took a look, created an account and was posting and following people within seconds, it was just really, really smooth. Again, that was not the case (for me) with Mastadon, where it took a while to figure some of it out, and it all just felt a bit fiddly and complicated.

Much like Lemmy in fact, after leaving Reddit - but again there was enough of a swell of new people shifting as a mass that it felt like it was worth the hassle.