Why did Canada pick Canada flag?

The current Canadian flag, known as the Maple Leaf or l'Unifolie, was chosen by the Special Flag Committee after a nationwide contest to replace the Canadian Red Ensign.

The flag, designed by a historian named George Stanley, was selected after a unanimous vote by the Special Flag Committee on October 22, 1964. It was approved by a vote of 163-78 in the House of Commons on December 15 (after much debate) and the Senate on December 17.

HM Queen Elizabeth II signed a royal proclamation on January 28, 1965 officially naming the Maple Leaf as the flag of Canada.

The new flag was first raised on Parliament Hill on February 15, 1965 in the presence of the Prime Minister and Governor-General. Simultaneously: the Canadian Red Ensign was lowered at the United Nations in New York, Canadian diplomatic offices abroad, and all Canadian ships around the world and replaced with the Maple Leaf.

February 15 is celebrated nationally as National Flag of Canada Day (or simply "Flag Day").