Political Parties are Private Organizations
While I happen to believe asking someone who participates in a primary or convention for the purpose of selecting that party’s nominee is a good idea, that’s not the point.
The Republican and Democratic parties are private organizations. In our free society, any individual or group of individuals may set up a new political party and go through the necessary hoops to get on the ballot. That’s why there is a Conservative Party and a Liberal Party on the ballot in New York. That’s why there is an American Independent Party. Unfortunately, most people don’t really understand or comprehend the ramifications of the Parties being private organizations.
It could be either a smart or a stupid decision to limit participation in the selection of a Party’s nominee for local, state, or national office. That’s something those who are members of the Party can argue over. But make no mistake about it, local, state, and national party leaders aren’t dummies. They have specific political goals in mind, but they also know that having goals is meaningless if you can’t win.
Most folks active in political parties hate so-called "open primaries" where an individual can vote to select a Republican nominee for Congress and then jump across the aisle and vote to select the Democratic nominee for State Attorney General. The reason they hate such "open primaries" is that if you are a Republican and your candidate for Congress is the incumbent who is assured of getting the nomination, you will likely vote to select the weakest Democratic nominee for Congress. Open primaries have inherent conflicts and do not serve the public well.
Conventions, contrary to popular opinion, are often the best vehicle for selecting party nominees. Primary voters are fickle. One stumble in a speech, or a temporary current event may cause undereducated voters to pick a weak nominee for their party. Those who generously volunteer their time to work, go door-to-door, stuff envelopes, put up signs, and stand out in the cold on election day are usually the most well-read and well-informed voters. They not only understand issues, but have specific philosophical reasons for supporting candidates of their choice.
The argument against primaries (closed or open) is to eliminate uneducated and independent voters from choosing the nominee of your party. The argument for conventions is that they more often select nominees who actually believe in something and have an agenda. These types of nominees give the public the best, most clear-cut choices in the general election.
The convention approach tends to be a more republican process when selecting political candidates, while the primary route tends to be a mobocracy which can result in poor choices by uninformed or mischievous voters.