Because to maximize profit you keep on selling until marginal cost equals marginal income(MC=MR)
Some Key points
MC is the increase in total cost from producing one additional unit.
The MR is the increase in revenue from the sale of one additional unit.
One way to determine how to generate the largest profit is to use the marginal revenue-marginal cost perspective. This strategy is based on the fact that the total profit reaches its maximum point where marginal revenue equals marginal profit.
