Possible Answer for Why the MD Wasn't Called
OK, I am a physician AND a pilot (non-airline). Here is one possibility as to why the MD seated in FC wasn't called, nor apparently was any effort made to seek professional help... by the time the FO realized that the Captain wasn't just napping, it was already too late. If the FO had witnessed an "event", the FA would have been called and an immediate call for medical help would have been made.... all mainline US airliners have onboard automatic external defribrillatots. If the AED had been used within a few minutes after a witnessed cardiac arrest, it is possible that the Captain may have been saved.
So that is my take... I do not know how often one pilot or the other takes turns napping on redeye flights, but in this particular instance, the FO believed that the captain was asleep, and it is only within an hour of landing and 15 minutes of beginning the descent that he tried to awaken the left seat only to discover that he was unresponsive, not breathing and pulseless. Hence, no attempt to resuscitate.