Raycast is successful because it's an incredibly widely used and adopted integration layer. The official repo has thousands of plugins, pretty well maintained together, collectively, and there are thousands more out there. https://github.com/raycast/extensions/tree/main/extensions

KDE krunner has plugins. But there is not afaik a big tent ownership system like Raycast. And krunner is only going to attract KDE users. Raycast is mainly mac, but also, there's Vicinae which runs on a lot more systems, and it can run >50% of the Raycast plugins without a sweat, is designed to be drop in compatible.

It's also absurdly easy to write Raycast extensions. It uses web platform technologies that "just work", that don't have ornery toolchains and need lots of dev libraries installed. With KRunner you can probably use whatever library you want to do whatever you want, which is good, but hard, where-as with Raycast there's a pretty comprehensive and powerful API that offers a ton, that is well defined, and that other plugins will show you the way on. https://developers.raycast.com/