There really is no explaining it. On March 17th every year, much of North America adopts the green and becomes Irish. It's a reason for a party! And American's serve green beer and corned beef and cabbage on their menus, but that food and drink was not and is not a common Irish tradition.
Originally, most people in Ireland could not afford or did not have access to beef. Their food was most often boiled cabbage and potatoes. Green beer was unheard of then and you'd be thought a fool in Ireland if you went into a pub and asked for one today. So exactly how and when Americans adopted the tradition of corned beef and cabbage and green beer as part of the celebration is somewhat unclear, but here is what we do know.
The St. Patrick's Day celebration began in the U.S. in 1737 when the city of Boston decided to celebrate the day of Saint Patrick's death as the Irish had been doing for many years. The Boston celebration spread to other cities in America every year until the entire country, it seems, was celebrating the day.
It was in the late 19th century that corned beef and cabbage began to become more popular with the Irish emigrants in America and Canada, where both salt and meat were cheaper than in their native country.
So pretend you're Irish, and if you're not, you should still get out the green, put on a shamrock, and have a party with friends and family by eating corned beef and cabbage and drinking green beer!