There's no real reason a Linux mail server should be an order of magnitude more difficult to administer than an Exchange server
Job security.
More seriously, the GUI takes a non-trival amount of overhead to run, and those of us who can't afford new shiny hardware appreciate that. The vast majority of Linux sysadmins I know are more comfortable in a terminal anyway. Plus, the Linux approach gives you some more flexibility. I'd much rather have a steeper leaning curve than to be stuck to The Exchange Way.
More seriously, the GUI takes a non-trival amount of overhead to run, and those of us who can't afford new shiny hardware appreciate that. The vast majority of Linux sysadmins I know are more comfortable in a terminal anyway. Plus, the Linux approach gives you some more flexibility. I'd much rather have a steeper leaning curve than to be stuck to The Exchange Way.