1: I live in Outer Bumfuckistan, so if I want books I either have to order them or wait for the rare times I get into an actual "city". With my kindle I can get them instantly.
2: With a little careful watching I can get a lot of books for free or deeply deeply reduced. This takes the sting out of paying full price (or these days near full price) for trek books.
3: Space and storage issues.
4: The built in dictionary.