I love Kagi and am happy to pay for search engines that don't screw users over. However, because one must log in to use Kagi, all our search history is connected to a single account. Unlike Google, we can't just change IP's, clear cookies, open Google in Private browser window, etc to make it harder for Google and friends to track us.
I trust Kagi far more then Google to do the right thing but many of us who seek a private search engine would like to maintain our anonymity as much as we can when we can. There is very few greater ways to link someones online idneity to their real identity then with their financial info. Every Kagi user that pays is effectively linked to their real identity even if Kagi promises in the future to never change their stance on data collection. By the nature of search it's also very hard to verify if it ever does happen. To make matters worse, as Kagi grows in popularity, governments will know that all payed users of Kagi are connected to their real identities, what more could a government ask for.
I personally will be holding off on paying for any search engine, not just Kagi, unless/until a private option exists as well. A good chunk of others probably feel the same way if not at least uncomfortable with the thought. There is a benefit for Kagi as well because by accepting Bitcoin or Monero, those who own the coins will have a new service they can spend their money on which will attract even more users to Kagi. Accepting Monero is a great way to get privacy minded people that own Monero to spread the word about Kagi.
Minimize data collection/potential leakage points is the first step to privacy/anonymity.