It's because it is dismal and Eugen is right as well and there is evidence supporting that [0] which it is still true to this day with the up-to-date instances that are active and with users who have logged in at least once over a week.
3,064,237 total registered Mastodon accounts and only 192,384 users using it. That is 7% of active users using the platform and 93% of everyone else not using it. There is virtually no one to talk to or interact with on the platform.
This is enough to tell us that Mastodon is indeed a failure and has not replaced Twitter after 6 years of marketing itself as a Twitter alternative.