Trust: I already have to trust Mozilla to design, write, and maintain a minimally safe browser written in a memory unsafe language that also properly supports Ublock Origin just so the web doesn't appear like a bunch of blinking Las Vegas signs. If they can do that, they can probably also maintain an instance of a social network written in a memory safe language (and probably hack on it, too).

Bus factor: they have more employees than can fit into a single bus

Communication: whether its web docs or an article about some clever Firefox internal, Mozilla's communication is typically clear and fun to read. If something goes wonky on their instance, or even if some part of the Mastodon is wonky enough that it messes with the UX of their instance, their write-up will almost certainly explain things in clear language, even for a neophyte.

Laziness: what would you rate as the easiest and most obvious choice of instance for an American who knows nothing about Mastodon? Let's set up a time to discuss this over tea. In the meantime I'll go on the waiting list for Mozilla's instance and probably nothing bad will happen as a result.

Mozilla is a bit less random than most Mastodon admins, giving you more assurance about the quality of the service and its life expectancy.

i’m not sure i understand the question. why not? are we so far gone that having a choice between multiple servers seems like a foreign concept?