A political party is free to change the rules for nominating candidates however and whenever it chooses. It is free to nullify the decision of those voting in a particular primary. A political party is free to nominate whomever it chooses [and almost certainly multiple candidates for the same office it wishes should it choose].
Ultimately the party, not a judge, chooses whose vote matters and whose doesn't. Placing the imprimatur of the state upon a political party's process doesn't change that or make the process of candidate nomination little 'd' democratic. The people within a political party charged with making the rules for candidate selection are not elected or selected little 'd' democratically. The process of nominating candidates is not little 'd' democratic in any meaningful sense.
Whenever people talk about alternative voting systems, the consensus seems to be that it would be impossible to implement in the US. But why? What drives this obsession with choosing between two evils rather than choosing among several, where one's own views might stand a better chance?
But with regard to your question, the obvious explanation is that the voting method itself acts as a game theory attractor for a certain number of "viable" candidates, until a Nash equilibrium is reached. First-past-the-post thus eventually results in an entrenched two-party system.
This alone is ample reason for those two parties to resist any change to the voting method. Anything else might undermine their duopoly.
You cannot get elected here as a third-party or independent. Period. If you had a notion to buy your way in, it would be cheaper to pay off the second-place major party to run you as their candidate than to run in your own right. The only place you will ever see a third party on the ballots here is the line for presidential electors.
And for a variety of reasons, in a manner similar to Comcast and Time Warner, the two major parties often choose to not compete in certain areas, to the ultimate detriment of the residents. I write in because when I see only one name on the ballot, that is a mockery of democracy.
If the vote counters hate me, that is exactly what I want. I hate how they support a system that pretends to be democratic.
What is your evidence that they choose not to compete? Competing costs almost nothing if someone wants the job and enough people fill out single party ballots that they'd have a good chance.
I'm disappointed that you choose your protest method to be inconveniencing local volunteers in a manner that is even less likely to achieve results than something you compare to holding back the tide.
To run for a county office, such as Sheriff, Coroner, Treasurer, Commissioner/Constable, or School Board, as an independent, I would need 5000 verified signatures. To list a party affiliation, I would need about 36000. Based on the experience of the Libertarian Party in 2015, I might face a 60% rejection rate for signatures, meaning I would need to collect 12500, costing roughly $25000.
To keep my party listed as a "qualified party" for the next election, it would need 20% (!) of the votes cast in a statewide race.
The major parties do not need to collect that many signatures, which gives an innate funding advantage for campaigning.
And you might expect that the other major party might not run an opposition candidate for Sheriff in a county where the incumbents are heavily favored every election, but how about allowing the candidate for U.S. Senate to run unopposed? [0] What about three of the seven districts for House of Representatives? [1]
I don't tilt at windmills.
[0] https://en.wikipedia.org/wiki/United_States_Senate_election_... : "An independent candidate would have been able to challenge Sessions if at least 44,828 signatures had been submitted by June 3, 2014."
[1] https://ballotpedia.org/United_States_House_of_Representativ... : Districts 4 and 7 were completely unopposed; district 5 had only an independent challenger.