We engage with decentralization protocols a lot (many nerds love to) but make no mistake: in the end, the app needs to work and be easy to use. #deltachat is a cross-platform FOSS offering, with guaranteed security levels, close to user expectations ... major factors in driving adoption!
True, the email system is not necessarily better than #xmpp ... but please compare with http://chatmail.at which is fast, ephemeral and secure, with delta inherently being a peer-to-peer E2EE messenger :)