Because if you're on the kind of malicious network that's the reason to use encrypted DNS at all, then your connection attempts on port 853 will probably just get blocked wholesale. DoH is better since it looks the same as all other HTTPS traffic.
And you can still block ad and scam domains with DoH. Either do so with a browser extension, in your hosts file, or with a local resolver that does the filtering and then uses DoH to the upstream for any that it doesn't block.