Every instance defederation from my instance only makes me grow stronger.
You, to me: You poor misguided n00b. You unknowing child. You ignorantly cling to your instance, but I will show you the light, even kicking and screaming, I will guide you to enlightenment.
Me: Jondescension!
When I looked into installing mastodon, it was extremely resource intensive and required a bleeding-edge version of Rust, so I went with GnuSocial instead. That supports ostatus, but not ActivityPub, so it federates with fewer and fewer hosts.
I could install pleroma, which has some issues, but is a lightweight design I could actually support, but many mastodon users block all those instances anyway.
tl;dr Masto is hard to install and expensive to run. Partisans hate the alternatives.
Server run by the main developers of the project 
It is not focused on any particular niche interest - everyone is welcome as long as you follow our code of conduct!
My "shitty" instance is the hill I will die on!