Email by its very nature is asynchronous and was standardized in a time when "always-on" didn't exist, and it is provider-agnostic because of the standard protocol.
Email's maturity by the time that businesses all came online pretty much guaranteed it as the standard for business communication, and personal use was just a small step from there.
Expecting 20s RTT for email is a modern phenomenon, when you could expect replies on the timescale of days before.
The closest attempt to a standard for IM is probably XMPP, which providers have pretty much all ignored in favour of lock-in to their respective platforms.
>Expecting 20s RTT for email is a modern phenomenon, when you could expect replies on the timescale of days before.
I think the whole point of having email is for communication where you'd expect responses on this timescale (for reasons outside of communication protocol used). Some questions just take time. The immediacy of instant messaging puts pressure to respond quickly.
Even for personal communication, if I want to write a longer letter, I don't get to do it every day.
On that note, none of the current IM clients are suitable for long messages. Even your comment is too long to be comfortably sent in an IM. There's a whole infrastructure around emails that make them suitable for long messages: drafts (so you can work on an email over the span of days), editing/formatting, and so on.
Then there's flexibility of the message being a unit of communication, rather than the chatroom. Your response can selectively quote parts of a message, being sent to a a part of the group, be forwarded. It's not easy to do with IM's.
But in the end, I do think that the non-instantaneous nature of email is the reason why people use it: IM's for short, fast responses, and emails for "...I'll answer it tomorrow".
Email's maturity by the time that businesses all came online pretty much guaranteed it as the standard for business communication, and personal use was just a small step from there.
Expecting 20s RTT for email is a modern phenomenon, when you could expect replies on the timescale of days before.
The closest attempt to a standard for IM is probably XMPP, which providers have pretty much all ignored in favour of lock-in to their respective platforms.