I know I may get blasted for asking this question/stating my opinion on this, however I am curious what others think about this.
Basically, I believe that the only reason that dirt jumping still enters into the conversation of mountain biking is because of two reasons:
1. The bikes have front suspension forks and therefore approximate mountain bikes more than any other type of bike.
2. The dirt jumping niche hasn't quite grown large enough to justify its own media (magazines, etc) coverage yet so it continues to get lumped in with mountain biking since it really has nowhere else to go.
I know there are people who are really passionate about dirt jumping and, in no way am I belittling this particular cycling niche I'm just saying that it's not mountain biking and, in fact, is MUCH closer to being BMX than what most (I'm guessing) consider mountain biking.