I have found it mostly has to do with the care letters are written with. An e-mail is like talking in real life, in many cases someone responds to it right away and if there is any misunderstanding you can correct it right there with another e-mail. In many cases a letter is much more formal, you have to take a little bit of care to make sure the message is getting across right the first time.
Plus, since letters take quite some time to arrive at their destination, and double that for you to receive an answer, they tend to be much longer (I think that is why they tend to be; not 100% sure). I've never received an email that was longer than three pages. Whereas with letters, I've both sent and received some very, very long letters, and almost all were a joy to read and to write.
