THERE ARE AS many theories as to what's wrong with Beacon Hill as there are people who watch it. And people are watching it--in droves, all the while complaining about how terrible it is. The basic problem is that the show is boring and stupid. The reasons for that are many, and as sociologically complex as one wants to make them.

Beacon Hill, like its English counterpart, Upstairs, Downstairs, is a study of relationships between social classes. The English version takes place in a land ruled by a centuries-old tradition of aristocracy, and the Bellamys are only a little less convinced that they belong where they are than their butler is. The originators of Beacon Hill understood that knowing one's place and sticking to it was a particularly British obsession, and at least did not try to create what would most likely be only a pale American imitation of the British show. But all the problems most commonly associated with the show--the slow storylines, the confusing number of characters have their source in a strange sort of non-understanding of what it meant to be rich and Irish in Boston in the 1920's.

America is a land of upward mobility--we've all heard that since grade school. Every boy can grow up to be president, and Benjamin Lassiter rose from poor immigrant to power mogul in Boston. But if Lassiter were really as nice and paternal as Beacon Hill's scriptwriters would have us believe--the gently father saving his daughter's name or genially sponsoring a young boy as ambitious as he himself once was--it's hard to see how he could have made as much money as he is supposed to have. Nice people don't get rich, in this country or anywhere, and Lassiter is far from ruthless.

NOR IS HE, or any of his family, for that matter, threatened by the newness of their wealth or the insecurity of their social position. They are all amazingly, boringly secure in a world in which they have no historical place. No Irish, as has often been pointed out during the past few weeks, lived on Beacon Hill in the 1920's. Any Irishman who would dare to move there, even in a piece of fiction, must have some distinct goal in mind, some all-encompassing ambition that would overcome the unspoken but still strict social rules of the time, and of our time.

But for CBS television to depict an Irish family in the middle of brahmin-land as having to be biting and aggressive would be an admission by one of America's largest corporations that this is indeed a closed society with a rigid class structure. It is much more politic for CBS to push forward the benign version of the American dream now found in Beacon Hill. It makes for a pretty show, an optimistic show, one that shows off little but the talents of the costume designer.

The show's unreal setting makes the reactions of the people living in it false, and nowhere is this more clearly shown than in the relationship of the Lassiters and their servants. Tuesday's episode showed Mrs. Hacker, the housekeeper, pleading with Rosamond, the youngest Lassiter daughter, not to sack the new maid. That a servant would dare to confront her employer so boldly would be unthinkable in a household of the British or American aristocracy. That a woman so recently arrived in the position of having a housekeeper would not be threatened (as Rosamond was not) by this kind of display of gall should be unthinkable in this one.

If the servants and Lassiters were at each other's throats, then at least the show would have some spark. But CBS is avoiding all possible controversy, including even the merest hint of racial tension between the black cook and the rest of the Irish staff.

The result is a situation comedy only a little more exciting than the Doris Day Show. In fact, the last two episodes have ended with I Love Lucy's favorite device of a wife benignly but purposefully misleading her husband. Even the soap operas to which Beacon Hill has been compared face up to hate and abortion and murder and all the other joys of everyday life.

So why is everybody watching it, this unreal, disappointing show? Because people are curious and Beacon Hill has gotten more publicity than any other show of the new season. Because Upstairs, Downstairs has a lot of fans and people were hoping for some sort of good show to forestall withdrawal symptoms until the English show starts again in January. And because TV is a holding medium--a form of recreation for tired minds who don't demand very much in the way of truth.