The primary reason the Populist movement broke up after the election of 1896 was that Democrats adopted several Populist causes. The Democratic Party, under William Jennings Bryan, embraced issues that were central to the Populist agenda, such as the fight for bimetallism (the free coinage of silver), which effectively absorbed many of the Populists' key platform points. As a result, many former Populists joined the Democratic Party rather than maintaining a separate identity, leading to the decline of the Populist movement.
The other options provided do not accurately capture the main reason for the breakup of the movement.