▻ THE TROLLEY PROBLEM STATED
For the benefit of anyone not familiar with the problem :
THE TROLLEY CASE: Edward is the driver of a trolley, whose brakes have just failed. On the track ahead of him are five people; the banks are so steep that they will not be able to get off the track in time. The track has a spur leading off to the right, and Edward can turn the trolley onto it. Unfortunately there is one person on the right-hand track. Edward can turn the trolley, killing the one; or he can refrain from turning the trolley, killing the five. ( Judith Jarvis Thomson, "Killing, Letting Die, and the Trolley Problem," The Monist 59
▻ THE MORALLY PERMISSIBLE VERSUS THE MORALLY OBLIGATORY
I think you are probably right that there is at least a majority view among consequentialists, among those who publish anyway, that Edward may turn the trolley. That it is permissible for him to do so. I am not at all sure that the majority view is that he must turn it, that he has a moral obligation to do so. Thomson herself does not go beyond permissibility; she never claims that Edward has a moral obligation to turn the trolley. Anyone can check her article on that point: p. 207.
▻ WHENCE THE MAJORITY VIEW (IF IT IS ONE) ?
The trolley problem is usually so baldly described that under a principle, which most of us accept, not to harm or to do least harm, saving the five seems the natural choice. We are given no context, just absolute generality; as the situation is standardly described there are no morally relevant differences between the six people. They could be identical skittles for all we are told about them.
The problem is thus reduced to a crudely quantitative one of doing or causing least harm. 'Five over one - what is there to discuss ?' This thought comes easily to mind. After all, we all agree that it is a tragic dilemma, no-one thinks it does not matter that a presumably innocent person loses their life if the trolley is switched. But moral mathematics dictates the choice of five over one - so one readily thinks in light of how the problem has been set up. For the problem is described in such a way that this solution is difficult, for many impossible, to resist. If the choice of five over one is not invariant among consequentialists, it is (I should estimate) the majority choice but only because, to answer your question, the example is so under-described. Given a fuller and more realistic description, the consequentialist can take a different view and make a different choice.
▻ CRITIQUING THE PROBLEM AS UNDER-DESCRIBED
We should note how artificial the situation is. In the first place, it involves a tragic dilemma. Such dilemmas do occur but they form only a fragment of the moral life. Moreover, and this is more important, when we are faced by tragic moral dilemmas it is rare for us to have the bare minimum of information that the example provides. In real life there are nearly always other, morally relevant features known to us in situations of moral choice.
Even as the example is described, it is hard to imagine that we would not recognise (though this is blotted out) that some of the people are old, others young, some possibly known and dear to us and others not, one a nurse (which we can tell by the uniform), and so on and on. We are seldom behind the 'veil of ignorance' (to purloin Rawls' phrase) where the example puts us.
No matter, someone might say, 'everyone to count for one, nobody for more than one'. There's no need to deny this. Intrinsically no life is worth more or less than another. But for a consequentialist, this can hardly be the full story. We are to calculate (aren't we ?) the consequences; and to lose the life of a nurse may have socially harmful consequences than to lose the life of a hobo.
I am not defending consequentialism; I am simply spelling out its logical implications. And they do not involve an automatic preference for five over one.