It's the path of least resistance to achieve Musklessness. The second two of the positives you listed are actually negatives to the average Joe. Choice paralysis, overwhelming number of apps and servers, these are things that put people off even trying, especially if there are easier-to-use alternatives that are familiar and instant.

Mastodon is great, but it's not quite there yet in terms of convenience. Too much copying and pasting and clicking through to different instances in order to read old posts etc. It needs to be more cohesive in a way that doesn't require constantly leaving your timeline or going into the settings.

It's also the case that the Twitter diaspora who are famous tend to choose BlueSky, and that brings a lot of people along with them.

And it's also the case that Mastodon doesn't have much of a marketing campaign outside of word-of-mouth, whereas BlueSky does.