Studies have shown caucuses prevent a full representation of the people; only a select few are able to attend these long meetings (usually on weekdays), resulting in a minority opinion. This disadvantages working individuals, single individuals, people of color, and parents - all of whom have historically shown a representative turn out at open primaries, but have been underrepresented in caucuses.
Closed and semi-closed primaries additionally put individuals at a disadvantage.
While these may be satisfactory for local and state elections, elections for the president are open and so the primaries should likewise be open.