Pure text is not good enough for many kinds of books - you need to at least support images (for cookbooks, children books, travel guides etc.), monospaced, bold, italic fonts (for programming books), vector graphics (for illustrations in technical books that will good at every dpi) etc.

There's no strictly ebook format that gives you that so you can either spend engineering resources trying to define and implement a new, non-trivial ebook format from scratch, with non-insignificant possibility of getting something wrong.

Or you can re-use the most wide spread document format in existence. A format that gives you everything you might ever want, has evolved over more than a decade, is tried and tested and works and is implemented on every platform imaginable and has open-source implementation that everyone can re-use.

HTML5 is the lesser of any other evil you can come up with as a new format for ebooks. Amazon made the right decision.