It has a character limit, it has separated instances on which you really can't subscribe (at least I couldn't a few years back), and it is realtime feed.
So who is the target audience really? Big influential public figures with millions followers? - not possible due to federation
Small thematic communities or specific professionals, not popular with masses? - very problematic due to federations first (all need to get on the same instance), then due to character count (no longform posts by professionals), and real time, so that rare good posts of people you care about are simply lost in the feed.
Even smaller groups of friends? - none of the problems above are blocking, but now the question is why Mastodon specifically, instead of a more usable and friendly chat room in any of the messengers.