Why was the election of John Quincy Adams in 1824 controversial?

Many people know this as the "corrupt bargain". Essentially the election was between three candidates: John Quincy Adams, Andrew Jackson, and Henry Clay (who was Speaker of the House at the time). Although Andrew Jackson won the popular vote, none of the candidates met the constitutional requirement for victory. So the House of Representatives had to decide who was president. For some reason Henry Clay was automatically eliminated leaving only John Quincy Adams and Andrew Jackson as candidates. Henry Clay had previous discussions with John Quincy Adams. It is unknown if they struck a deal. But Henry Clay said he supported John Quincy Adams. And since he had a lot of influence in the House John Quincy Adams won. Also Henry Clay became Secretary of State. Furious Andrew Jackson called it the "corrupt bargain".