Why don't (some) grownups laugh at elephant jokes? Because they don't see the point -- the principles being violated are not matters that they care about or have emotional commitments to. Consider whether you laugh at these:
Q: How do you know that an elephant has been in the refrigerator?
A: There are footprints in the butter dish.
Q: How do you know that two elephants have been in the refrigerator?
A: There are two sets of footprints in the butter dish.
Q: How do you know that a herd of elephants has been in the refrigerator?
A: There is a Volkswagen parked in front of your house and there are lots of footprints in the butter dish.
This relentless, repetitive series of jokes are often not funny for many adults, but for many ten-year-olds and for some adults, elephant jokes are quite hilarious. Children are highly involved in actively constructing their view of the structure of the world. Just as everyone cares about, is emotionally attached to, whatever they are actively involved in doing, children have emotional commitments to the structure of the world. In the ten-year-old's world, it is a wonderful and essential feature of elephants that they are gigantic in size. Further, it is also a widespread attitude that one isn't supposed to dirty one's food; kids, especially, are actively being socialized into keeping food in the right place (on the plate or in the mouth) and their (often dirty) shoes off of things that need more considerate treatment. So there are a great many principles which children are emotionally attached to, which are violated in these particular elephant jokes: giant elephants are somehow small enough to climb around on a stick of butter, and at the same time, food is trampled on. These violations show how kids can find these jokes funny. At the same time, they are not so attached to the principle that they would be offended by breaking it: Few children have so great a personal investment in elephants being huge that they would be offended or personally threatened if elephants were tiny. Nor is the idea of someone else hypothetically stepping in some food a violation that cuts very close. Further, the cuteness of the answer, the difficulty of seeing anything wrong built into the question itself, and the banality of the question/answer format all help to make it so that the situation is clothed in normality. Finally, the surprise of the answer creates the simultaneous juxtaposition of the two affects in the minds of the perceivers.
For adults, on the other hand, the size of an elephant is more or less an accidental fact that might conceivably be otherwise, without changing the essence of elephantness. Further, the prohibition, ``Keep your dirty feet off the clean things in the house'' doesn't have the emotional impact with adults as with children. In any case, adults do have a model of reality with decades of sedimentary accumulation in which they may have lost their emotional involvement in such things as the size of elephants, or even the idea of stepping on a stick of butter - which might remain inconvenient, but is no longer grounds for teasing or punishment. At least for those adults that are not amused by these elephant jokes, the theory suggests that these violations no longer have sufficient affective impact.