Why did Jackson supporters think he should have won the election?

He had more electoral and popular votes.

Crawford and Clay made a deal to support Adams.

He should not have been eliminated from the vote in the House of Representatives.

It was widely believed that Clay, the Speaker of the House at the time, convinced Congress to elect Adams, who then made Clay his Secretary of State. Jackson's supporters denounced this as a "corrupt bargain."

He had more electoral votes and popular votes

Explanation:

Jackson supporters were furious when the House chose Adams. They thought that Jackson, having more electoral votes than the other candidates, should have been the next president. The election seemed unfair and against the will of the people.