Then it would be easier for an ISP to block encrypted DNS (by port number). It is better to masquerade everything as normal HTTPS to make blocking more difficult.
For most people, if you can't trust your ISP, you have bigger problems.
For people who can trust their ISP, why should we all by default be affected by the fact that the Mozilla developers seem to all live in a non-free or non-democratic country.
Maybe they should instead focus on fixing the US political system that results in their current situation, rather than trying to use technical means to solve political problems.
I have a very specific case for this: in the days before and during the referendum for the Catalonia independence (Oct 1s 2017), all the spanish ISPs where blocking access to the websites related with the referendum, using DPI to look for the SNI hostname. One of the main reasons to enable DoH in FF is to enable the encrypted SNI feature https://miketabor.com/enable-dns-over-https-and-encrypted-sn...