I think because if one person held a monopoly on (let's say clothes) it provides less variety for everyone as well as essentially they get all of the revenue and smaller (family owned and such stores) businesses go out of business or bought by the bigger businesses. (Also think of the board game Monopoly, when someone owns a lot of places that cost a lot and people land on it, they pay a lot. Then someone runs out of money and has to sell the property, meaning less of a chance for them to make money and those with money now buy the available land, increasing their chances of getting money) (So essentially the rich get richer and the poor get poorer and there is a large wealth disparity)
Hope this explanation helps. If not, I could try to elaborate.
