Because they're oftentimes generated at an anonymous whim, or are the preferred form of mass-mailing efforts. They're free, which means the sender only sacrificed the few seconds it took to type some incoherent, illegible babble about some issue, and you can't emphasize anything over email.
Meanwhile, with a phone call, you not only tie up a staffer, but the staffer gets an earful of a living, breathing person who decided an issue was important enough to take the time out of his day to call, pay long distance, wait on hold, and finally yell at someone.
Just like how a boss' reprimand has more effect when done verbally, so too does a voter complaining to a Congressthing.
Thank you. That makes sense.
During the Impeachment fiasco, I did a lot of calling to my "critters" in DC and Atlanta.
I kept hearing that for every telephone call they get, they estimate there are about 300 other people who care about as passionately about a given issue, but, for whatever reason, can't manage to call- too shy, too cheap, can't find the time, etc.
So view your call as having a 300:1 multiplyer.