This is 100% the issue. When Mastodon started, it was like when IRC started -- one big server you could talk to just about everyone on. These days, it's like IRC was in 2000 or so -- many different servers you could talk to your small group on, but you'd end up joining a whole bunch of different servers to get your reach across, and unlike IRC, that really gets in the way of things like re-tooting and other broadcast issues.

It's not so much that it's a problem so much as that it makes it a poor choice for a Twitter replacement. Is Statusnet (a la identi.ca) still a thing?