Because no one met the constitutional requirement for winning, the election was decided in the House of Representatives.
Historical context:
In the presidential election of 1824, Andrew Jackson won the most popular votes (43%) and won the most electoral votes also. But since no one had attained a majority of electoral votes, the decision was given to the US House of Representatives, which named John Quincy Adams as president. Jackson accused Adams and Speaker of the House Henry Clay of what he called "a corrupt bargain."
Four years later, when Jackson ran for president again in 1828, he pulled no punches. The 1828 presidential campaign (which was won by Jackson) was nasty, dirty affair in which all sorts of personal "mudslinging" attacks were publicized between the candidates. That set a precedent for the sort of negative politics we've been seeing ever since.
