Ask HN: Why XMPP failed and SMTP didn't?

I was born too late to witness popularity of XMPP as a decentralized instant messaging protocol. When I started to be a "conscious" user of the Internet, such services already became centralized, so I don't really remember those times. As I follow the development of projects like Matrix, I wonder what has made SMTP such a stable protocol that has stood the test of time and what is the real reason we don't use XMPP (I mean, we do using Whatsapp, but as a part of closed ecosystem).

Is it only that XMPP was overcomplicated and if managed better it would survive? Or is it something else?