Google Translate:
First of all, the ActivityPub system is not suited to something like Misskey, which flows at lightning speed.
It was originally designed to connect blogs, small-scale SNSs, and wikis.
How can it handle the TL hell where tens of thousands of requests fly in per second?
It’s based on the idea that it would be nice if various small services could send each other updates, so it’s quite costly.
Might be a privacy feature, I checked out the NodeInfo of one of the 0 active users instances (which I checked and did have active users) and it had “null” in the relevant fields. Only the total users were given.
So the data just doesn’t seem to be there for those instances. Considering it works for some and not for others, I assume it’s opt-out for some reason.
That said, FediDB gives 11.3K MAU for Misskey.