Because the judicial branch is not answerable to anyone. At least the executive and legislative branches can veto each other temporarily and apply some pressure. But when the Supreme Court fails to act there is nothing and no-one to force it to.

In English common law there is a writ called mandamus which can be used to force a public official to discharge their duty ie do their job. The US would've inherited this writ, for certain. Perhaps it's still there but these days never used?

It defies commonsense that a little court can directly interfere with presidential policy on which he campaigned (successfully). I think a better approach would be that where a little court wishes to make a ruling of this magnitude, it lacks authority to do so and must apply to a higher court for the necessary authority.