Why don't other isps use it? Is it due to the extra load placed on the server?

Well... POP3 is older. Standards take forever to get replaced. IMAP is a much newer protocol. It's a little more complicated in its design, and because its encourages you to keep the mail stored on the server, it's likely to take more disk space for a large number of users. I think that might have a little to do with it.

You can check POP mail from several different places too, you just have to tell your clients to leave the mail on the server. By default most clients delete after checking, but it's not required.

Oh, I use Eudora 5.1 as well. I wish it had more than 7 or 8 labels for message categories/colors, but other than that it's great. One of my email accounts supports IMAP but I just use POP3 because it's simpler to manage.