Why did Stephen Harper resign as leader of the Conservative Party after the 2015 election?

Stephen Harper resigned as leader of the Conservative Party following the 2015 federal election, in which his party was defeated by the Liberal Party, led by Justin Trudeau.1

During the 2015 campaign, Harper faced significant challenges that undermined his attempt to win a fourth consecutive term. These included criticism regarding his economic stewardship—compounded by a struggling economy and a history of budget deficits—and the distraction of a high-profile scandal involving Nigel Wright and Senator Mike Duffy. Additionally, several of his campaign statements and policies regarding immigration and religious expression drew intense public and political criticism.1

Ultimately, the Liberals secured a majority government with 184 seats, while the Conservatives fell to 99 seats. Following this electoral loss, Harper stepped down as party leader, though he remained in the House of Commons until August 2016.1