So according to Eugen, he's been using XMPP during a time when people let their desktop PCs run all the time, which is ideal for XMPP's requirement to maintain an active connection from sender to recipient in order to actually deliver messages.
But when the world transitioned from desktop PCs to mobile phones, XMPP's requirement to stay always on was just not practical, and the world moved on from XMPP and onto other platforms.
The only way people used XMPP was through Facebook and Google Talk, but the mainstream really didn't have a strong appreciation for XMPP.
According to Eugen, Email is still going strong because everyone knows how to work with it
Effectively, embracing, extending, extinguishing is just not a thing for email yet.
