This is a technical thing. The court decided that Nebraska and Oklahoma can sue the Federal government for failure to enforce Federal law, but cannot sue another state.
No that's not it. The whole majority decision is one line long:
"The motion for leave to file a bill of complaint is denied."
That's it. No four justices voted to take the case. That decision sets no precedent and they declined to provide any reasoning which could even be persuasive in terms of trying to predict how they will react to future leave-to-file motions.
We can speculate as to why they decided not to take it, but we can't know for sure because they didn't say.
"The motion for leave to file a bill of complaint is denied."
That's it. No four justices voted to take the case. That decision sets no precedent and they declined to provide any reasoning which could even be persuasive in terms of trying to predict how they will react to future leave-to-file motions.
We can speculate as to why they decided not to take it, but we can't know for sure because they didn't say.