"Why has ''liberal'' become a dirty word for so many Americans today?"
Liberal used to be a good word here and abroad. It meant to be liberated from government, or monarchy. American liberalism meant anti-communism, smaller government, lower taxes. Now it means the opposite, and conservatives now stand for all those things.
You brought up an excellent point. Now the Libs want to be known as "progressives". Sheese. The devil by any other name is still the devil.