Here's the quickest answer: https://help.kagi.com/kagi/why-kagi/kagi-vs-competition.html#kagi-vs-duckduckgo

There are a ton of features that Kagi has and other search engines (including DDG) don't, you can see the range of these in the features list on the sidebar of the above link, especially the many detailed ways you can customize your search results: https://help.kagi.com/kagi/features/website-info-personalized-results.html 

Kagi Search also actively downranks sites with lots of ads and trackers in the results and promotes sites with little or no advertising, shows if a site is paywalled from the results page before you click, allows you to filter out AI generated images in image search, and much more.

The best way to see the real difference is trying it for yourself - explore the settings and personalizations, see how far you can go with Kagi Search vs. others in the market, all without ads poisoning your experience.