This is because writing letters and making phone calls takes (sometimes marginal, but still) more effort than sending an email.
After all, it's not much effort to vote for his opponent at the next election.
Writing a physical letter introduces a barrier of effort which separates a two minute slap-out-an-email rant from a carefully composed and considered letter. Imagine if commenting on YouTube required writing a letter? The quality of comments would increase by many orders of magnitude.