Congress and the Supreme Court have colluded to provide joint jurisdiction between SCOTUS and District Courts for most of the cases where the constitution gives SCOTUS (and only SCOTUS) original jurisdiction.
SCOTUS has even turned away acting as trial court in State v. State cases. See Arizona v. New Mexico, 425 U.S. 794 (1976), where the Supreme Court denied Arizona leave to file the complaint.
That case also cites some of the earlier precedent. The gist of the justification for not taking original jurisdiction as the constitution demands is that SCOTUS is too busy and too important to take on trial work, what with all the evolution of the law in the United States.
Additional justification is that SCOTUS sucks at fact finding and conducting trials, so the litigants get a better product in a lower court.
As a counterpoint, one should be aware that Congress can increase the size of the Supreme Court, and that there is no reason the Supreme Court must hear any case "en banc" (all judges hearing the case and contributing to the decision). Trials are decided by single judges, and most appeals are decided by a panel of three.
Thanks for the info.
I read the link Mr Rogers provided and now see where SCOTUS indeed does relinquish power to the lower courts.