Transatlantic passenger boats largely ceased to operate by the early 1970s, as commercial air travel become more affordable. Even before that, passenger volumes for transatlantic ocean liners had been on a steady downward trend since the early 1950s, as those that could afford it increasingly used airplanes for transatlantic travel.

Edit: Sources :http://www.lastoceanliners.com Books: The Last Atlantic Liners by William H. Miller. Most of the info the on website is supposedly pulled from this book.