After the English invasion of Ireland, wearing green could get you killed as surely as wearing red, white and blue in Iraq could today.
The Irish, Catholics and Protestants alike, fought under a green banner against the "Orangemen" occupying their land. Green became a symbol of the proud Irish rebel and shamrocks a political statement of Irish nationalism against English occupants who outlawed their language, education and faith.
St. Patrick's Day is a celebration of the survival of a race that faced genocide from famine and oppression and overcame religious persecution, economic oppression and exploitation here and abroad. The Irish came to the United States seeking freedom, but instead were greeted with racism, forced to celebrate Mass in secret and compete for positions in slavery.
When the Irish immigrated to the U.S. because of the potato famine, most Irish were thought to be no good drunkards and they were pretty much not excepted and ridiculed. When St. Patrick's Day would arrive, few wore green because of this oppression and few didn't so they pinched those who didn't wear green for lack of pride and it was passed down.