Honestly, it's a great question! While kbin is open source, making userscripts allows you to iterate on changes and get updates out very quickly. It's also much easier to start making userscripts than it is to contribute directly to the project, since you only really need to be familiar with JS, HTML, and CSS. To contribute to kbin, you need to be familiar with PHP and Twig, and set up your own instance for testing your changes.
Plus, while kbin is open source, not every single change will be merged, and there will likely always be features "missing" from it. Userscripts also give you a ton of freedom and flexibility in how you use sites, so you could make crazy changes that aren't remotely possible to merge into it.
minnieo
GeekFTW