Because if you take away competition between business then you take away the incentive for people to improve their work or up their production. As a result of this you get horrible productivity, shitty products and poor service and not only do your own citizens stop wanting to spend their money, but nobody in other countries wants to buy your shitty products, so you lose out on trade to free-economy states. The end result: circulation of currency is low within the nation, no money is coming in from other nations, employment and growth stagnates and the country grinds to a standstill.