According to the Oxford Dictionary of National Biography, certain people are simply "letter writers", and that is enough to get them in The Big Book. Others at least did something else too to get in there but that fact that they wrote letters seems to be special somehow. For example, Agnes Maclehose, as well as having a stupid name, was their Life Of The Day on the 22nd, and is noteworthy for being a "letter writer and poet".
Well, I've written letters. Over the years I've written quite a lot of 'em. I have to say, it doesn't make me feel particularly special.