Why did political parties switch to national conventions to nominate presidential canidates?

Political parties switched to national conventions to nominate presidential candidates primarily to create a more democratic and transparent process. This shift allowed for broader participation from party members and delegates, moving away from earlier practices where party elites or congressional leaders made decisions in private meetings. National conventions also helped unify the party by showcasing candidates and platforms to a wider audience, fostering a sense of party identity and mobilizing support for the general election.