After installing the Tor browser, the user's default search engine is https://duckduckgo.com. However, it make more sense for the user to search with DuckDuckGo's onion service. This way all the routing happens over Tor, making the user more anonymous and private. So why does the Tor browser come with the default search engine of https://duckduckgo.com?
It is currently unknown whether their onion service would be able to handle the load the entire Tor Browser user-base would put on it.
It is also not known what the performance drop (or possibly increase) would be.
For anyone else: there has also been some discussion on reddit: https://old.reddit.com/r/TOR/comments/m52jeo/why_doesnt_the_tor_browser_use_duckduckgos_hidden/