Perhaps it comes from a feeling that Congressmen are too busy to read their mail, that one letter won't make a difference, or that their letter won't be answered properly. Speaking for myself, I can say flatly those notions are wrong. Like most members of Congress, I have staff help to process my mail, but all of it is answered and comes to my desk for signature. As for impact, there have been many instances where a well-worded and persuasive letter either changed my mind or caused me to review my opinion.
Letters are important.
Unlike previous eras in the Congress, sessions are longer, issues more complex and vital to the lives of all Americans. Dear Congressman Rhodes. Your Representative and your Senators cast your vote. Stick with them. You don't have to type, but write clearly.