First, tradition is an imperfect term. It is not inaccurate, but it is a catch-all that glosses over a complicated, political process. No one has ever been satisfied with "tradition" as the answer to questions like...
Why are presidential nominees selected in this way?
Why are Iowa and New Hampshire always first?
Why aren't all primaries opened/closed?
Why do we still have caucuses?The list goes on.
Again, no one is ever satisfied with that answer -- tradition -- because it borders on the tautological: the rules are the way they are because those are the rules. Additionally, it always leads to the very obvious follow up question, "Well, why?"
The answer ultimately lies in the fact that rarely is an alternative put forth that can garner widespread enough support among convention delegates or national party committees or state parties or the state themselves as separate groups or collectively to be implemented. It is not that there are not alternatives. There are plenty of them, but what they lack is consensus. To the national parties what they lack is certainty. To the states/state parties what they lack is flexibility.
The trouble with presidential nomination reform is not tradition. Instead the problem is that reform is a complex effort at coordinating widely ranging interests within and sometimes across diverse political parties. If that were not enough, add to the mix that state governments, often in partisan conflict with at least one party, control important aspects of the process (eg: when primaries are, who can participate in them, etc.).
Those competing interests often make bringing about fundamental, sweeping changes (or even incremental ones where there is enough overlapping interest) to the system impossible. Such a set up does not tend to produce directives from the national party to close all primaries, for example.1 To the contrary, with so many interests involved, the likely outcome is more incremental and smaller in scope; like the RNC decreasing the proportionality window from a whole month in 2012 to just two weeks in 2016. Incremental changes -- goals -- are more achievable, but are the very types of tweaks that are most likely to maintain large parts of the status quo.
...or tradition, if one will.
1 Closing or opening primaries cuts to the very heart of what political parties are. They are private organizations that can, under the first amendment, freely associate with whomever they choose. But who gets to choose who gets to associate; to participate in a primary election? Under the above idea, it would be the Republican National Committee (or the delegates at the national convention) making that determination. But what about Republican state parties that would prefer a more open system that allows the party to potentially draw more voters into its ranks? That was the basis of the Tashjian case. Or what about the open primary states that are controlled by Democrats in states that Democrats in control of at least one part of the decision-making apparatus within the state? Does a national party push those state parties toward closed caucuses? Do they offer some sort of incentive/penalty structure to gain compliance? What? The more one digs, the more complicated it gets. The more questions arise.
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