Answer (with explanation):
Because of the fact that you have to start from nothing and turn it into something. In math, they give you the equation where you just need to solve. In history, you just got to remember the facts. But in writing, you have to brainstorm, think up how you will write the prompt, set up a whole strategy to persuade your professor (if it was a persuasive essay), and also do everything in a very tight schedule. That's the hardest of the hardest when you try to juggle your subjects and run into something uneasy to be taken care of in a few minutes.