Mozilla provides a clear policy for how you get your resolver onto their list. US ISPs (the DoH resolver is only enabled by default in the US) could obey that policy and apply to be added to the list.

But it seems like none of them have done that. Maybe the policy terms are objectionable? Let's see:

"Only aggregate data that does not identify individual users or requests may be retained beyond 24 hours."

But how will the poor ISP make extra money selling DNS query information?

"When a domain requested by the user is not present, the party operating the resolver should provide an accurate NXDOMAIN response and must not modify the response or provide inaccurate responses that direct the user to alternative content."

An ISP that obeys this can't put up advertising banners or sell search engine redirects when you typo a name - they'll have to actually earn money providing Internet service instead.