Sell orders are a quick way to show supply of goods. Supply is very simple and easy to see. Demand is more difficult to know.

The law of supply and demand is the theory that prices are determined by the relationship between supply and demand. If the supply of a good or service outstrips the demand for it, prices will fall. If demand exceeds supply, prices will rise.

Crafters can produce goods and attempt to sell on auction house via sell order, but if demand is not high enough for that item to sell, the crafter is basically wasting their time and effort.

Customers can ask in world or trade chat WTB (want to buy) and a specific price. If both the customer and crafter agree on any specifics of the transaction, then they can quickly transfer items for gold, completing the sale. That is the main issue, that this conversation must take place. Compare that to the reverse of this example. The crafter makes the item and lists it on the auction house for that price; the customer finds it on the auction house buy order and completes the purchase. No conversation with the crafter is necessary, since the customer wants that item at that price and completes the sale. Customer receives goods and is happy. Crafter receives gold and is happy.

Buy orders do not yet exist, but would show demand for an item. As demand increases, the price of the item would increase, as more customers create buy orders for that item at increased price, until it matches the listed buy orders for the item. At that point, the auction house would automatically 'sell' the item from the sell order to the 'buy' order, essentially removing the need for a customer to ask WTB in chat. Crafter receives gold and customer receives item.

The example is for when supply and demand match, but other scenarios would also exist, for lowered/increased demand or supply. Make supply and demand automatic. ie simplify crafting/supplying customers with items.