It protects privacy
Duckduckgo is a search engine that doesn’t collect data about its users. Some of the people who are concerned about online privacy shift to Duckduckgo. (See Why Do People Care About Online Privacy?)
It became an option in Safari
The fact that Apple made Duckduckgo one of the options when choosing a default search engine gave it a strong push.
Doesn’t provide personalized results
The fact that Duckduckgo doesn’t provide personalized results makes people feel safe as they become reassured that the search engine is not collecting their data.
Duckduckgo has some extra features such as Password Generation tools and tools to check whether a website is up or down. The extra features could have attracted some more users to the search engine.
People who don’t like Google
Some people don’t like Google’s practices. Those people usually support Google’s competitors whenever they have the chance.
According to an article on Business Insider, DuckDuckGo provides good and accurate results. This could have motivated some users to shift to it.
All search results appear on one page
DuckDuckGo displays all search results on one page. When a person scrolls down, more results show up. Some people might have preferred that feature over the traditional paged results other search engines provide. (See Why Internet Cookies are harmless?)
To try a new thing
DuckDuckGo is the latest search engine appearing on the radar of many internet users. People who like to try new sites could have given DuckDuckGo a chance.
The ability to search other sites
DuckDuckGo provides a feature that can allow people to search in famous sites such as Amazon, Reddit or Ebay. Many people like that feature and it could be one reason they are using DuckDuckGo.