At that time, there wasn't much commercial interest in having e-mail fragmented. For messengers, no company wanted to give up their users and their things. For a while, multi-clients like Pidgin, Miranda or Trillian sort of fixed that for users, now we pretty much seem to be back to darker ages in that regard, thanks to tons of messengers being based on "one mobile device per account" models.

You see some of that with e-mail and development around it as well (webmail for more features instead of clients and protocols, ...)