Thousands of petitions reach the Supreme Court each term and only a small fraction are heard. The selection process is discretionary, and the criteria have little to do with correcting individual injustices.
Review is granted, not owed
Almost all cases arrive by petition for a writ of certiorari, which asks the Court to take the case. The Court is under no obligation to grant it.
A denial is not a ruling on the merits and sets no precedent. It leaves the lower court's decision standing for that circuit and nothing more.
This distinction is routinely misreported. A refusal to hear a case says only that four justices did not think it warranted the Court's time.
Four votes open the door
By longstanding internal practice, four of the nine justices must vote to grant review, while five are needed to decide the case.
The gap is deliberate. It lets a minority force consideration of an issue the majority might prefer to leave alone, which keeps the docket from being controlled entirely by the majority bloc.
Justices sometimes vote against granting a case they would win, on the reasoning that a bad vehicle can produce a narrow or damaging ruling.
Circuit splits drive the docket
The strongest argument for review is that federal appeals courts have reached conflicting answers to the same legal question, so the law differs by region.
Uniformity of federal law is the Court's central institutional function, and an unresolved split means identical conduct is lawful in one circuit and not in another.
Petitions are drafted to emphasize this, and opposing briefs typically argue the split is shallow, illusory or likely to resolve itself without intervention.
The vehicle has to be clean
Even an important question is passed over if the particular case carries complications: disputed facts, procedural defects or an alternative ground that could decide it without reaching the issue.
The Court prefers a record where the legal question was squarely presented and fully argued below, since it reviews decisions rather than developing facts.
Because of this, a question can circulate for several terms while the justices wait for a better case in which to answer it.
Screening happens before the justices read
Most chambers pool their law clerks, dividing petitions so a single memo on each circulates rather than nine independent reviews.
Petitions that interest any justice go on a discuss list considered in conference. Everything else is denied without discussion, which is the fate of the large majority.
The system exists because reading every filing individually would consume the term. It also means the initial screen is performed by recent law graduates working to a standard set by their justices.