Probably a very polarizing question.
On the one hand, having most of the users and communities on LW causes technical issues (see this post), and also gives the LW staff too much power over Lemmy as a whole.
On the other hand, with 18k MAU on LW out of 47k (https://fedidb.org/software/lemmy/), every community listed there has a much higher chance of visibility compared to an alternative hosted on another instance
Or there aren’t enough specific ones. If you go to Join Lemmy and you are presented with a number of general purpose instances, you are likely to pick the largest and only later realise the problems that entails and switch to another instance.
If you are a Trekkie or read books or game or program then it is easy to pick one. Ditto if there is an instance specific to your country (I should know).
If you look at Mastodon (which is more developed and has a wider and deeper selection of instances) you can see that these niches instances do well and I think we need to encourage more here.