The opinion says that the March 27 petition deadline is too early, especially given winter weather in South Dakota. It also says the 2.5% (of the last gubernatorial vote) is also too high, given the historical record of few minor parties qualifying. The Reform Party did not qualify in South Dakota in 1996; the Green Party has never qualified; the Natural Law Party never qualified; the New Alliance Party never qualified.
The state defended the March 27 deadline by saying that deadline is necessary to give new parties their own primary in June. But the opinion says there is no state interest in requiring new or small parties to nominate by primary. It says, “In our two-party dominant system, the Republican and Democratic Parties often have more than one candidate for each political office and thus need to run in a primary election where the registered voters of each party must choose their candidate. But Defendants have not explained why this rationale should apply to new political parties.”
In our very close elections, whether or not third parties can get on the ballot and compete in presidential (and other) election contests can make a real difference, even if these parties usually have little chance of winning.
You can find the court’s opinion in Libertarian Party of South Dakota v. Krebs at this link.