The Populist movement began to break up after the election of 1896 for several reasons, but a key factor was that Democrats adopted several Populist causes. In that election, the Democratic Party, led by William Jennings Bryan, embraced issues that were central to the Populist platform, such as free silver and agrarian reforms. This absorption of Populist ideas into the Democratic platform attracted the support of many Populists, leading to the decline of the independent Populist party.
While other factors, such as changes in the economy and internal divisions within the movement, also contributed to its decline, the successful incorporation of Populist demands by the Democratic Party played a significant role in the movement's dissolution after 1896. So, the answer is primarily because Democrats adopted several Populist causes.