I suppose the idea would have to be that there are significant non-commercial sources of pdf and text documents that someone might want to read on a kindle (think documentation, academic papers, a collection of notes, the kind of things that aren't available through amazon).
However, those same kinds of documents are increasingly distributed as epubs, because PDFs are horrible unless you target one specific output size and resolution (like physical paper), and text simply doesn't offer enough layout flexibility. Epub is the open standard, and anyone who wants layout flexibility uses it. Anything else, including a lot of kindle ebooks, are typically converted from an epub original.
It's not a technical challenge. KF8 is nearly isomorphic to epub.
