I suppose that emails are bombarding us all day, everyday, and people are inclined to give them a lesser importance than a handwritten/typed, personally addressed letter.
There was originally 'Netiquette' (codes of behaviour on the internet) and it was said that you have about 48 hours to reply to an email.. any longer is seen to be rude. I have to admit that I'm rather bad at replying to emails.. but for no obvious reason. Though, I am aware of it and am trying to improve!!! :) :)