In order to get on the ballot, a person has to meet several requirements. One of these, in some states, is gathering signatures of supporters. Another, in all states, is to be eligible for the office sought.
What is inconsistent on the part of the courts is that they are willing to entertain a remedy for one criteria (signatures), but not another (qualification to hold office). The "standing" rationale says that no voter is affected if a name wrongly appears. That answer doesn't distinguish WHY the name wrongly appears, just that ineligible names on the ballot are a harm with no right to a remedy, unless you are an opponent on the ballot; or that the venue to remedy the harm is electors and Congress, not the courts.
Probably because the signature requirement is controlled locally and is a requirement to be put on the ballot. Local laws
Eligibility under the US Citizen NBC requirement is a Federal issue.