An HTML solution is not without a lot of difficulties itself. HTML can be rendered easily enough with an embedded browser--but you probably have sometimes observed the difference in rendering between Firefox and IE...very similar to the RTF problem.
See Difficulties in achieving WYSIWYG at
http://en.wikipedia.org/wiki/HTML_editor
Editing is a big problem...not so hard to implement if your target user population likes to input the markup tags in a text editor....but are embedded WYSIWYG HTML editors as well developed and as available as embedded RTF?
Otherwise it's the big guns...Word processors or specialized HTML editors like MS Frontpage (still installed with Office?), Nvu (nolonger developed?), Mozilla Composer, etc.
So it seems to me - User expectations are high and the technologies to met them are flawed.