For very general reasons small things outnumber big things. In the case of services this tends to mean that most users are using one of a very small set of providers despite the many smaller providers outnumbering the few big ones.

Wait until Lemmy spam becomes a problem. Consolidation will happen at that point and smaller instances will simply be restricted to being read only.

Reddit has the same problem, anybody can make an alt and spam random BS on small or large subs. Moderators just deal with it.