That's because a lot of software displays HTML content either online, internally generated or from a file, and the easiest way to handle it is to use the HTML viewer ActiveX control that is included with IE. There is a similar control that uses Firefox instead, but the last time I tried it there was still some missing functionality.
That makes about as much sense as requiring you to have Adobe Reader installed because the documentation is in a pdf file before you can install the software. Why not make note of the fact that a certain type of functionality requires a particular type of display mechanism but lets the install proceed anyway? A separate source for the necessary file would then be up to the user. I do not need the Adobe Reader to properly display pdf files, nor do I need Netscape or Earthlink to access html files either.