Storage was my biggest reason. I begrudged having books taking up so much space when I re-read them so infrequently, but I couldn't bring myself to pass them on just in case I did want to re-read them.
Close second though was convenience. I enjoyed leafing through books in the store and browsing for my next read, but often the books I were looking for were not in stock. eReaders solved that issue, although the problem moved to not available in ebook form for a few years
but the instant delivery and HUGE store browsing plus ability to read other peoples opinions of a book, especially reference books, made up for the diminishing problem of books not yet available as ebooks.
That's what made me start reading on a PDA. I moved to eInk once it became available to solve the issue of reading outside :P