There are at least three reasons why the Populists’ attempt to form this coalition was ultimately unsuccessful.
First, it is not necessarily easy to form a coalition between farmers and factory workers. While some people might think that both of these groups are working class and therefore share the same interests, values, and goals, that is not really true. Factor workers are urban people. Many of them in those days were immigrants. Farmers are rural people, most of whom were not immigrants. Farmers worried about things like the prices that railroads charged while workers worried about working conditions and pay. These differences meant that they did not really see one another as natural allies.
Second, we have to remember that this was a time when racism was the norm in the United States. Whites all over the country, but particularly in the South, saw themselves as superior to African Americans. They wanted to maintain white supremacy. They did not feel that it was right for them to ally with people who were their inferiors. In addition, many Southern whites did not want to abandon the Democratic Party for fear that this would harm the power of white supremacy in the South. (The Democrats were the party more associated with white supremacy in those days.) Thus, racism and the desire to maintain white supremacy helped to weaken the Populists’ attempted coalition.
Finally, the Democratic Party saw the Populists as a threat. Because of this, they tried to attract people from the Populists. They took some of the Populists’ ideas as their own. By doing so, they attracted people who might otherwise have joined the Populists. This, too, weakened the Populists.
These three factors all contributed to the ultimate failure of the Populists’ attempt to form this coalition.