Qwant claims like DuckDuckgo to be a privacy focused search engine with european servers and their own indexing. So what is the reason Tor doesn't show it as a search engine? Thank you for your help!
Installing addons makes your browser more easily identifiable, therefor they decided to go minimalist on it. Think about it this way: if 80% of Tor users used DDG and 20% used Qwant, by using Qwant you're making it 20% easier to finger print DDG users, and you're making yourself 80% easier to fingerprint by fragmenting the search engine base.
As for why, you would need to ask the developers directly.
If you want to add you own, please see the answer in "Special search in tor". No extra addons are needed and it will not increase your fingerprintability as long as the service truly down not track users. If it does, then it is the same as any other non-private search engine.