Kbin is about 3 months old whereas Lemmy is about 3 years old. Kbin simply wasn't ready for growth, and still has a few major feature shortfalls. Lack of API access is a big one.
And those who were stubborn enough to keep looking at all options despite what could easily feel overwhelming to a non-tech user.
I am not very tech-inclined, so trying to understand the fediverse as a whole felt like very deep water, and lemmy really didn't do any job at all at explaining what it was I was committing to by choosing an instance when I didn't even know what instances were and had trouble finding out.
Since Lemmy was and is by far the most mentioned alternative and I found it too anxiety-inducing, it's mostly stubborn desperation that brought me to kbin. Which says nothing to how much I'm genuinely enjoying it here, to be clear, I was just relieved to have a simple option at that point.
Being made anxious by a new platform isn't great and I would guess that most people who didn't like the experience they had with Lemmy didn't bother clicking other links that would take them to the same fediverse. They're likely to assume they won't enjoy that one either, and resign themselves elsewhere.
So kbin naturally got fewer users just by word of mouth, and then the necessary brief isolation didn't help either when people were still getting comfortable and testing out different accounts. I don't particularly mind it other than those rare times I'm accidentally excluded, either by a question addressed only to Lemmy or their recent version of r/place disappointingly being incompatible with kbin.
I was initially put off by the UI of Lemmy that I encountered when I first went to the Lemmy site. I was a little confused as to which instance to join. That's when I stumbled upon kbin.social and that's where I landed my new account. Overall I am most comfortable here.
Since joining, I've encountered Lemmy posts that take me to their instances proper, and the formatting looked different, more like here just with a different colored background. Overall, Lemmy instances seem okay, I just like it here better.
Maybe it's the overall familiarity with the instance, calling main topic pages "magazines," the microblogging option, etc. Lemmy's resemblance is a little closer to Reddit, so that might account for why people decided to go there instead of a kbin instance.
Because Lemmy is older and already has a lot of established communities. Kbin saw the larger growth rates though (in the context of the recent Reddit drama). Kbin also currently lacks native mobile apps, and a lot of people browse this type of media form their phones.
Regarding the articles: tech journalists all use microblogging sites like Mastodon, so for them Kbin's microblogging integration is a major advantage. Also, IIRC Kbin made it easier to follow certain people, like industry leaders, while Lemmy is more focused on communities.
Personally, I really dislike microblogging, so for me it was a major reason not to use Kbin. I think there may be a large silent group that feels the same way, but I haven't seen any statistics on this.