Which goal of the populist party led to its early popularity?

The "Coinage of silver" or "Free-Silver Movement" was the reason. This was an attempt to make silver  the official currency in order to make the value of the money more than what it represented. During the time of 1892 to 189 there was an agrarian-populist political party which was called the people’s party or as commonly stated the Populist Party.

Coinage of silver or Free Silver Movement was the goal which led to the early popularity of the Populist Party. The America of late 19th century is witnessed the presence of unlimited silver coins. The Free silver Movement was initiated by an Act of Congress in 1873, excluded silver dollars from the list of permitted coins. The people who were supporting the free silver movement included owners of silver mines in the west and farmers. The farmers were hoping that increased currency would help in raising the price of their crops. At that time, Silver become a sign of economic justice for people of America.