If I had to take a guess, I would assume that it is because word count is a little more manageable to look at than character count. For instance, the rough draft of the book I am writing is 119,595 words or 668,947 characters. Imagine, for a moment, what the character count must be for Way of Kings, which, if I remember Brandon mentioning on Writing Excuses, is somewhere around 250,000 words. What is easier to mention to other writers, or your agent, editor, or publishing company? 250,000 words or 1,400,000 characters?
Of course, this is a pure guess, but it the most logical one I could arrive at, based off of my own personal experience. I tried to find an answer on Google, but the Internet has failed me in the five minutes I hunted.