Not liking iTunes or wanting something with better sound reproduction would be the main reasons. Says the man who owns an iPod.... Nobody else builds them with enough space for all the music :'(
I've yet to find a sound quality issue with iPods that a decent pair of phones won't cure, without the need for EQ, too.
And I've tried quite a few 'phones, too, so I know just how much sound quality can vary.
The Cowon players are bloody expensive for just listening to audiobooks, which might as well be in mono, for all the quality that's needed.
A 16Gb Nano would be perfect, and it doesn't need iTunes for audiobooks, as others have pointed out, although I don't have any real issues with it.
If it's on a PC, I understand that version can be a bit flaky, however. The Sony's look pretty tasty, too.
When can we have music players with 128 or 256Gb of solid state storage? Preferably 256.