The second reason is there are still things they need to fix. For instance, they never got around to merging in a calendar. My biggest gripe about Eudora is when someone sends me a reminder or meeting announcement, I have to cut and paste to a different app (usually Yahoo Calendar). And while their junk filter is getting better, I still find myself in the junk folder every morning plucking out important email.
I hate to say this, but MS Outlook is going to become my email software. They have fixed the show-stoppers. Used to be if you had an Exchange server account, POP mail was either disabled or just didn't work. That's been fixed, and now I can put all my POP mail accounts into Outlook. I've got a pile - Earthlink (both mine and my parents'), gmail, comcast and four on howeird.com. Work email is usually an Exchange account.
Also, Outlook is the email program of choice for all the PDA-to-PC and cell phone-to-PC apps. And Yahoo-to-PC-to-phone, now that I think of it.
Should I wait till Office 2007 is out? Nah, easier to upgrade. Especially since I'll probably be working at MS then.
When I first discovered email, I was new to Silly Valley, using a dumb terminal and a 150/300 baud modem. My first email account was on The Well, and the only way to store messages locally was to print them. I had a cheap dot matrix printer, I think.
Over the years I changed email addresses frequently. Working at TeleVideo, I had a company Internet connection and email account, and for the next decade or so, I considered email to be a mandatory benefit, like dental coverage.
Email was still text-based, but since TeleVideo made terminals and CPM computers which could display simple ASCII-based text features like highlighting, reverse video and blinking, small email apps started to pop up. The first one I remember was elm. Actually, I used elm as a Usenet news reader, and someone on Usenet (Dave Taylor, I think) reminded me that Usenet messages were in standard email format, so I could use elm as my email client.
Some time after I was using an IBM compatible at home, I tried pine as my email program. But while it was hyped as easier to use and was more graphical, I just didn't like the look and feel. I felt the same way about the Windows version which most folks know as belonging to my alma mater, the University of Washington, but I know its origins in an entirely different way which I won't discuss in a non-friends-locked posting.
Moving right along, in the early days of Windows, there were tons of email programs introduced into the public domain, either as freeware, shareware or as crippleware demos. I tried several, and the two finalists were Eudora and Pegasus. I used them side by side for a while, but found Pegasus had too many features which I had no use for, so I kept Eudora, and have ever since.