There might be a broader base of deployed email servers. It is easy to get an email account somewhere, and also easy to host your own email server. EMail might

There might be a broader base of deployed email servers. It is easy to get an email account somewhere, and also easy to host your own email server. EMail might have been stress tested more, because of all the spam mail. I am not sure how common it is for XMPP to spread hundreds of messages per user in a short amount of time.