The depressing prospect of having to choose between Jimmy Carter and Ronald Reagan this fall forces a Job-like question. Why? Oh Lord, why?

The answer seems to be that baffling national troubles have further emphasized the exaggerated weight attached to personal qualities -- especially trust -- in choosing a leader. The rub is that the private virtues are currently at odds with the qualities necessary to master public problems.

The personalization of the presidency runs far back through many roots. Franklin Roosevelt contributed his buoyant confidence, Truman his grit, Eisenhower his heroic stature, and Kennedy his glamour and martyrdom. Johnson and Nixon did their bit in a negative way. They fostered the illusion for close identity between individual maladjustment and national health. They spawned the camp of psychohistory.

Television, especially the recent surge, which has reduced most of the rest of us to boutique journalism, also figures importantly. The camera gives intense focus to personal traits. Toward institutions and ideas it turns a blind eye.

Then there are the reforms adopted as safeguards against abuses of the "imperial presidency." Changes in the rules of the nominating game have worked to heighten personal, as distinct from party, organization and finance, and to prompt the building of momentum by early entry into small states where a handshake makes the difference.

The novel, and intrinsically hard, nature of the problems now confronting the country fortifies all those tendencies. Persistent inflation at high levels is not a disease familiar to Americans, nor one for which there is an agreed prescription. Never before has the country been dependent upon foreigners for a vital resource. The Islamic fundamentalism that makes for so much of the trouble in the Mideast, and with Russia and the allies, is a curve ball thrown by history that nobody knows how to hit.

Confusion, laced with suspicion of chicanery by government, the oil companies or other supposed authorities, is the normal reaction to the new difficulties. Thus cut adrift from their normal moorings, voters instinctively turn to traditional standards -- especially personal confidence.

Jimmy Carter measured up in every way. He is a moral man who believes in his God, cherishes his family and saves his money. As president he has constant access to television. His personal organization was established in the 1976 race, and he began running for reelection when he descended from the domestic summit at Camp David last July.

The acute sharpening of inflation, the energy crisis and the difficulties in Iran and Afghanistan only gave him a final fillip. He became the good man trying to do his best.

Ronald Reagan is, as much as Carter, the perfect, gentle knight in private life. Being on television is his profession. He too has had a private organization in being since the 1976 campaign.

A spot of trouble obscured his chances when he held himself aloof in Iowa. But as soon as he resumed normal personal campaigning, the homilies about making America respected abroad and getting government off the backs of business proved sure-fire stuff with Republican voters.

The losers tell much the same story as the winners. Kennedy enjoyed a great name and took salient positions on the big issues. But he never overcame -- indeed, if anything his campaign deepened -- the mistrust following Chappaquiddick.

John Connally is perhaps the most able political leader in America, with a sure sense for projecting authority. But he never overcame the mistrust inspired by his wheeler-dealer past and his party switch.

As for the two other Republicans still in the race, George Bush was seen -- on national television -- to tense up after he took an early lead. John Anderson was seen -- also on television -- to think himself superior, as he indeed is, to other Republicans.

Unfortunately, the personal qualities so critical in winning and losing nominations bear little relation to government. A strong sense of the past, and of the overwhelming importance of American commitments, comes crucially into play in foreign policy. The capacity to take large views and make broad decisions is essential to dealing with inflation and the energy problem.

Whatever their virtues, Carter and Reagan are not famous -- even among their own supporters on their best days -- for those qualities. They are ahistorical and inclined to miss the forest for the trees, not to say the bark. So much so, that it can be said that a Carter-Reagan race will at least spare the country one recent misery -- overblown expectations.