It has to do with how eBooks are formatted.

PDF files, for example, are electronic versions of print works. They are formatted for a specific paper size. If you are trying to read a PDF on something smaller, the PDF often ends up being unreadable - either because the font is too small or because you end up endlessly scrolling (which is painful on slow eInk screens) to read everything.

Just because a book is available in electronic format doesn't mean it will be as readable as the paper version.

Getting back to the Kindle in particular, the screen is significantly smaller than a normal tech book. So imagine your tech books shrunk down to that screen size. The text will probably come out OK, but will the code examples fit on the screen? Will the images be clear now that they are smaller?

Grabbing a reference book from my bookshelf at random - the Perl Cookbook - If the book were shrunk to the size of a Kindle screen, the code examples would become very hard to read, as would any table of output. So the value of having such a book on my eBook reader would be very low.

As for the iLiad, imagine what the Kindle would be like if the screen were significantly larger and it used WiFi instead of EVDO and, unlike the Kindle, supported open file formats.