Why did the House of Representatives decide the winner in the election of

1824?

No candidate won the majority of electoral votes. According to the 15th amendment, in the event of a tie the House of representatives decide the winner.

Contingent election.

Explanation:

In the case of an Electoral College dead lock or if no candidate receives the majority of votes, a contingent election is held. This is where the House decides the winner. This is actually supported by the 12th amendment.