The key difference between everyone else in a company and Sales is that Sales is where the money comes in, directly. It's approximately trivial to point at a sale, see how much money it made, and then who made the sale. So commission is a natural compensation structure for salespeople. For everyone else at a company, their individual contribution to the company making money is a lot more diffuse, and any metric you might be tempted to try and put a commission on is at risk of being gamed. Whereas "how much $$$/worth of stuff did we sell" is pretty much THE metric by which we judge a business as a whole.