For the Supreme Court to review a state court decision, there must not be an independent and adequate state law ground of decision. (e.g. A judgment that is partially based on a violation of a state Equal Protection Clause.)
If a state court decision rests on two grounds, one state law and one federal law, if the Supreme Court's reversal of the federal law ground will not change the result in the case, the Supreme Court cannot hear it.
If the state grounds are fully dispositive of the case and do not depend on federal case law interpretation, federal review would be pointless because regardless of how any federal issue is resolved, the state law grounds would support the state judgment.
But if the state grounds are not adequate to support the judgment or are not independent of federal law, the federal courts will hear the case.
If the state court decision turns on federal grounds and is from the highest court in the state, the Supreme Court can review the state court decision.