Because most mastodon instances are running off donations, and have a relatively small user base.

The kind of people who use Mastodon are substantially more likely to be heavily invested in the technology and the vision, and thus more likely to donate.

Expand that out to the billions of people who use social media, and you have a funding problem.

Not to mention the much lesser need for moderation due to more homogeneous and well-intentioned micro communities and substantially lower rate of bots, which all means less “staff” you have to pay too.

It’s not a matter of minimum viability, it’s a matter of scale.