Why did the House of Bourbon lose the Spanish throne in 1931?

The House of Bourbon lost the Spanish throne in 1931 primarily due to the actions and political associations of Alfonso XIII. His decision to authorize the military dictatorship of Miguel Primo de Rivera in 1923, and his subsequent association with that regime, severely tarnished his reputation among the public and political classes.12

Following the collapse of the dictatorship in 1930, the monarchy faced mounting pressure from an alliance of Republicans, Catalan politicians, and former liberal monarchists who had united under the Pact of San Sebastián to overthrow the crown. The king’s attempts to return to a constitutional government were viewed as insufficient and came too late to restore his authority.12

The definitive end occurred following municipal elections in April 1931, which resulted in a landslide victory for Republican and Socialist parties in Spain's major cities. Faced with the prospect of civil war and widespread street demonstrations, and having lost the critical support of the army, Alfonso XIII departed Spain on April 14, 1931, though he notably refused to formally abdicate his rights at that time.123