I joined a bunch of instances (including kbin) with a bunch of different usernames, and have mostly stopped visiting kbin. The default browser interface isn't as good: no collapsing comments, unintuitive displays of what magazine/community you're on (thread links weirdly prioritize telling you which instance hosts the community rather than which of that instance's local communities it is), etc.
This UI/UX stuff matters, I think. After all, a big part of the reddit migration was prompted by users being forced off of their preferred interface.
The UI for both is just reddit. And Kbin has a PWA that functions like a native app. The only issue I've found (on my phone at least), is some formatting issues with certain buttons.
The Kbin PWA is fine, but the native apps really give Lemmy a leg up. There are some genuinely fantastic apps like Connect and Liftoff that Kbin can't match right now.
I hate the way they handle images. I was browsing kbin on my phone, and they show images 3 or 4 times as wide as tall. They put a tiny little thumbnail of the original image that fits in there, on a background of a blurred version of a strip of the image. It looks awful, and you can't even click on it to see the image. You click once and it takes you to the thread, and you need to click a second time to see the image. Then back a couple times to get back to where you were.
tl/dr, kbin had me first, and I left because I didn't like the UI.
Kbin seems to have some trouble viewing content in lemmy, but lemmy seems to have no trouble viewing kbin content, something about federated server being incomplete. I don't know enough about federation to diagnose the problem or who is responsible, but lemmy account sees more stuff so that's where I went.
I actually prefer kbin.(social). It hasn't had nearly as much downtime and errors and whatnot as I've experienced on lemmy. Which is remarkable since Lemmy has been around for a few years whereas kbin is fairly recent.
Like, there are always growing pains, so I get it, and with all the influx of new users there can be issues, so not a big deal.
I think Lemmy's more popular mostly because for whatever reason it's the catchall name for the fediverse right now. IDK if there's any like (very mild) astroturfing going on, or the name is just catchy, or what.
It's the name that's catchy, and word of mouth when the Reddit drama happen, people weren't mentioning Kbin that much.
Also Kbin sounded like a website for uploading pictures of programming code that you want to send to another programmer. It's a very utility sort of name, where Lemmy is a very cosmetic sort of name.
If want an unprovoked opinion of a new join, I picked Lemmy because it was a simple word I could pronounce. I'm not saying kbin would be difficult to figure out, but having a simple name to pronounce is an easy "Welcome" sign to the newbies.
I should probably check around kbin at some point and I might even like it better. I just found what worked for me and I'm hesitant to change anything right now.......okay you motivated me, I'll check it out this weekend.