Pages on your web site are automatically in html format, but many ezine editors archive in plain text appearance. The format is html, but looks like plain text. This is usually to make it easier for other ezine marketers to pick up these articles in their text-based newsletters, and not to have to delete the html coding.
Personally, I find it much more appealing to read archived articles that have a bit of life to them, with basic html. For exampe, simple bolding of headings and sub headings. And, when your article is copied into a text editor, the bolding doesn't show up anyway. So why not use a touch of class! and use html perks.
The most important thing I find very strange about most archived newsletters is that they do not have Google AdSense ads on the web pages. Well why not? Is it because you just didn't think of doing it, or assumed it's unnecessary.
Or are you assuming no one reads Google AdSense ads from your archives? If you ever go to Article directories to read or copy their articles, you will see that every page has Google AdSense ads. They make their living this way.
Your archived newsletters, those containing good content, are in that same class. My back-issues are read a lot, my archives are very popular. So I do take advantage of this fact and have added AdSense ads to them. It only takes a few minutes to do this after the mailing is done. Believe me, it's worth it for the extra revenue potential.