Listing Requirements
Companies wishing to have their shares listed, or quoted, on a stock exchange have to meet certain requirements set by the exchange. Their market value, revenue, the number of their shareholders, the size of their profits and the number of years they have existed have to be above a certain minimum. After the shares start trading the company has to continue to meet these and other requirements; if not the company will be de-listed.
See also: http://www.nyse.com/regulation/nyse/1147474807344.html