How steep demand and supply curves are is purely determined by price elasticity. That is, how sensitive the quantity demanded and supplied are to changes in prices. A very flat demand curve is highly elastic as a small change in price will cause a large change in the quantity demanded (graphing it out is the best way to understand). Alternatively, a very steep demand curve is highly inelastic as a change in price will cause a relatively smaller change in quantity demand. Thus, monopolies profit from this knowledge by increasing the prices of inelastic goods (steep demand curve), where hiking up prices won't eat into demand too much. Although demand will drop, the extra revenue generated from higher prices allows the monopoly to earn extra revenue. If the the good had been elastic however, the increase in prices would've diminished demand by so much that not even the extra revenue would make up for the large drop in output.