One technical advantage of Delta Chat over XMPP is delay tolerance. You can put Delta Chat on top of any store-and-forward network and get all the features: encryption, groups, audio messages, WebXDCs etc. In XMPP to use OMEMO you need a publish-subscribe service and to share files you need HTTP upload server or connection to the same network to transfer a file over Jingle.
For example, it works over HF radio by copying emails using UUCP: Hermes // rhizomatica
You can run email over landline phone network (PSTN).
We previously (2021) built a prototype of Delta Chat running over Yggdrasil using yggmail, completely without email servers: GitHub - deltachat/AndroidYggmail
You can also run email on top of http://www.nncpgo.org/ or https://winlink.org/ or https://reticulum.network/ if you put some effort, but you don’t need to change Delta Chat for this to work.