This is in part a cultural thing. Within GNU, many are hostile toward non-plaintext e-mail and might even ask you to resend plaintext. But we're a group of people that live and die by text. Many hackers are extremely efficient in processing text. Adding document formats like HTML into the mix muddies things unless you have very specialized tools for dealing with it.
And then you have people who start using formatting semantically, e.g. colors.
Now, with all of that said, Emacs does a remarkable job of rendering a subset of HTML. For example, I have my mail client configured to always ignore HTML alternatives unless they're the only option. In that case, colors and tables and such are all rendered as they should be. Inline images might be supported if you use a graphical version of Emacs (Emacs can definitely display images, I just haven't tried in this context); you also have the option to open it in an external viewer, like a web browser.
And then you have people who start using formatting semantically, e.g. colors.
Now, with all of that said, Emacs does a remarkable job of rendering a subset of HTML. For example, I have my mail client configured to always ignore HTML alternatives unless they're the only option. In that case, colors and tables and such are all rendered as they should be. Inline images might be supported if you use a graphical version of Emacs (Emacs can definitely display images, I just haven't tried in this context); you also have the option to open it in an external viewer, like a web browser.