Because in a nation with the size and diversity of the US, advocates of Socialism seem willfully blind to the fact that there are people who would disagree with the implementation.
On a tangential thought, why can't we have both? This is the future. We should have a customizable, user-friendly government and economic structure.
Maybe Citizen A prefers free enterprise, so he opts for a capitalist citizenship. He works for a living, pays a low level of taxes and receives little help from the government. Citizen B prefers a socialist citizenship, so he receives a stipend and single-payer health care, and a large portion of his earnings are sucked away in taxes. Citizen C lives in a shed down by the river and prefers the anarchist option; he pays no taxes and expects nothing from the government, and is indeed excluded from any laws that affect only himself. He can't hurt other people, but he can drink and do drugs all he likes.
There is no reason, in today's information-rich society, why every citizen can't choose among several different legal and economic options, all of which combine to form the government system of their country. All it would take is a database and a form for citizens to choose their option.
These systems -- capitalism, socialism, communism, anarchism -- they are not each others' enemy. They are different options that are all superior in some way, and people should get their choice in the way they want to live.