Final answer:
The adoption of the presidential primary system by most states was influenced by the development of the direct primary system, Progressive Era reforms, the rising influence of political parties, the election of 1824, and the needs for greater democratic participation in candidate selection.
Step-by-step explanation:
Several factors led to the adoption of the presidential primary system by the majority of states in the U.S. The process began with the direct primary, allowing party members to vote directly for candidates instead of through delegates at conventions. South Carolina adopted this system for statewide elections in 1896, and Florida began using it for presidential nominations in 1901. The Progressive Era further accelerated the shift towards primaries and caucuses.
Presidential nominations, once dominated by party leaders, increasingly moved into the hands of party members. The change reflected a desire for more democratic participation, reducing the power of party regulars and allowing for a broader electorate influence. Additionally, the growth of political parties and their influence on the nominating process, dissatisfaction from situations like the election of 1824, and the natural evolution of the political process were contributing factors to the shift toward primaries and caucuses. The change was aided by Roosevelt's showing in the 1912 Republican primaries, as well as the eventual acceptance of primaries as a key part of the nominating process.