They have to install that somewhere and configure it to use that instance by default.

It’s not super complicated but also not trivial.

Edit: It’s not about “supporting” a frontend. It’s about “offering” them.

Depends on the front-end from what I heard

It’s more code that can be abused. Balancing availability and security is never a straightforward task.

It’s usually just an extra container and a little vhost config. There’s very little to it, hence why lemmy.ca has all of them as options.

Because people have different tastes. Be glad you have choices.

Edit: It is not complicated. Setting up Tesseract for my instance was very easy (one new podman container and a change in the caddy config). Some instances might offer less frontend options most likely because hosting something costs money and requires extra maintenance so they don’t want to host something that they think people won’t use.