Because McCarthy and his colleagues did not want to find any. And because the Congnitive Dissonance Psychologists behind the operation wanted you to believe that therefore there Were NO and are NO COMMUNISTS, in The United States of America, at that time.
As if!McCarthy: Failed to identify any communists ...Also they were guaranteed to not identify any actual politically active communists, - by deliberately not looking in the right Places. They investigated Hollywood. That's a place of free artistic expression where an actor can even put on airs by Method acting, to utterly convince an audience of a role. One might for instance portray having extreme left-wing views, in a role. It's all totally unsubjective.
To find communists you'd look in the POLITICAL HEARTLAND, the TRADE UNIONS, the workplace Shopfloor etc
McCarthy had no intention of finding any communists. Therefore the American people would be assured there were NO COMMUNISTS in AMERICA. --- except those hiding in plain sightResource:
http://www.conservapedia.com/Joseph_McCarthy