Ironically, Adobe, the company that created the standard for portable documents, is not suceeding in the eBook space. Adobe Acrobat Reader runs on Windows, Macintosh, and Linux, and has been a popular way to send formatted documents across the Internet. However, it was not designed with PDA's and portable readers in mind. The result is that eBooks in this format either use up too much memory, or that they don't look right on smaller screens. One of the reasons there are so many standards is that publishers and distributors are trying very hard to prevent piracy.
Software is incredibly easy to copy.
With a book in plain text on a PC, you could send 100 copies per second to computers all around the world. You can't purchase an eBook from Barnes and Noble and email it to your friend - it just won't work on their computer. In the same way that mp3's make record publishers nervous, an eBook without copy protection is something that would make a book publisher think twice about releasing their titles in an electronic format. Copying a book into computer-readable form, while impractical, is possible. Even if the eBook encryption was perfect, you will always find someone willing to go to their local library, borrow their favorite book, and import it into their PC at home with an inexpensive flat-bed scanner.