Either because your email account isn't formatted to receive HTML, or more likely because the person who sent it to you didn't format the email address in HTML. In other words, they've simply typed
[email protected] without using the computer languge that turns that address into a clickable link, or what's known as a "mailto". (I can't show you what those characters are here because this site will turn what I write into a clickable link!) This explains it better than I can and shows you what I'm wittering on about:
http://hotwired.lycos.com/webmonkey/teachingtool/m
ailto.html