Where in the Constitution does it say the Supreme Court has to be “ready” to hear a case?
As for the Court's readiness to hear a case, it refers mostly to a cautionary consideration that as the country's ultimate judicial authority, it is usually best for the Court to let a particular legal issue develop through several cases that are fully litigated to reported decisions in the Courts of Appeal. Doing so permits the Court to have the benefit of a range of views from other judges and courts before making a decision that -- good or bad -- may last for decades.
As it happens, the Court will often take up several cases at the same time but then select the one with the best facts and legal arguments to carry the Court's main opinion. And modern communications and litigation strategies and the intensity of American politics often quickly bring major issues to the Court's doorstep for resolution.