Just like MrsD said. They're high in fat. They're still good for us (unless a person is allergic, of course), but they simply have a lot of calories.
Not to worry. A little is actually quite a lot. I use nuts on salads and the like. I buy the crushed and pieces... first of all they're cheaper that way, and second of all they spread nice and evenly over my food =)
Most of the time I don't even need a whole ounce of nuts (a serving) I can have a nice salad with only half an ounce (15g)! That still gives me the great nutty taste, but comes down to half the points