In a court hearing on whether Jim Bennett and the United Utah Party should be placed on the ballot, assistant attorney general David Wolf warned the judge that if Bennett does get on the ballot, it could open the door for other candidates to gain last-minute access to the ballot.
If you are the Republicans or the Democrats, that could be a bad thing. They don't want competition. Another candidate might emerge strong enough to threaten the two parties' stranglehold on elections. If you can keep them off the ballot, locking out any competition, you can preserve your stranglehold.
People should be free to challenge the existing parties. They should have the right to start parties in opposition.