The Irish Tricolour flag represents the country of the Republic of Ireland, 26 counties of the island, which in fact doesn’t include the 6 counties that St Patrick mainly operated in. In fact, it can be quite offensive to many people living in the place St Patrick mainly operated in, due to a civil conflict.

St Patrick also has his own flag - a red saltire on a white background - which deliberately represents him.

Making St Patrick’s day a festival of Irish Nationalism specific to the Republic of Ireland makes zero sense, and using a flag which only represents part of the island, especially since many Americans are also descended from Ulster Scots in Northern Ireland.