They're being targeted for constant DDoS attacks. Could be multiple reasons. But the takeaway should be to spread out evenly through the fediverse. Don't all pile into .world (as I post this on my .world account).
Lemmy Explorer can help give a big picture view of what's out there, both in instances and in communities. Kbin instances are also viewable, you have to select them from the top right menu.
Been on kbin.social for a month and it's been pretty solid. Note it's a different platform than lemmy (designed to read both lemmy and mastodon), but does a great job posting to and handling the content from lemmy platforms. There's growing pains here too, but the uptime has been good.
I think that the dev and some volunteers at kbin.social has done a lot of work on scaling up that instance, but even so, kbin.social will probably hit hard-to-fix scaling issues at some point. It's also a big instance, like lemmy.world.
I'd been looking at another (presently small) US-based one-person kbin instance, was planning to hop over if it stayed up to help spread load, but it looks like it went down.
I suppose that once a number of instances have a track record of staying up for N months and with their uptime records established, it'll be easier to figure out what instances are good alternatives, are likely around for the long haul, and which ones will vanish in the wind.
You're right, and one of the reasons I decided to start a community here was because @ernest has done a fantastic job laying out the plans for scale at kbin (both the instance and the platform as a whole) thus far - detail here. Thus far he's the only instance owner I've seen with a real roadmap, and actively applying for grant funding to cover scaling costs. It's good to see - kbin is definitely his baby and he takes great care of it.
Regarding other small kbin instances, you might look at kglitch.social. @kglitch has made some frontend changes that bring it more in line with Mastodon and Lemmy, and I haven't seen an outage yet.
The big reason for being a target is the size. Why attack smaller ones with less effect? Like making a virus for anything besides Windows. But maybe the long term benefit is the movement of some to other instances, balancing out the loads.