Because the Indians were simply in the way of a land grab and were not, for the most part, specifically targeted for extermination by the USA or European powers. It is true that there were a few who took such a view, but the majority view of the invaders was that the natives were simply in the way and had to be removed, one way or another. The Holocaust, on the other hand, had the explicit motive of extermination. Doesn't make one any better or worse than the other, but it is valuable to distinguish among the various types and motives for genocide. Saying that "genocide is genocide" and demanding that they all be called "Holocaust" is as foolish and futile as saying "disease is disease" and they all be called "cancer".