Holocaust and genocide are basically synonymous.

The natives acted aggressively when they didn't accept our imperialism and instead tried to impose their grains upon us.

Straight from the first sentence of wikipedia:

from the Greek ὁλόκαυστος holókaustos: hólos, "whole" and kaustós, "burnt"

It guess it just applied more because, well, the cremation chambers and whatnot. (I'm not sure if the Native Americans or any other genocide relied so much on fire, so this suggestion could be void.)

Because it would force American exceptionalists to confront some uncomfortable facts.

Imperialism happens get over it and let's face it, imperialism in this case was for the better.

Of course it's easy to say that on the side of the Imperialists. It's not all smiles and sunshine when you're the victim of it. In some cases "getting over it" isn't as simple as "oh okay whatever". See Ireland.

_{I'm talking about it in general.}

The conquering of Native Americans isn't put on the same level with the Nazi holocaust because conquering a territory was kind of the thing to do back then. The British Empire was pretty big on that. But the Nazi holocaust was different because the world thought they'd reached an era of peace. They'd just finished the War to End All Wars, and most powerful nations had stopped their rampant imperialism and conquering. Globalized economy was replacing the need for territorial power struggles. So when the Nazis suddenly began waging war, it took the world by surprise. And the nature of the holocaust was much different too, because it wasn't the Nazis conquering a land and driving away its people, but the systematic capture and execution of several ethnic groups for the sake of killing them.

I'd also contest the claim that there even was a Native American genocide. The overwhelming majority of deaths were from diseases the Europeans carried over, not from aggression on their part. Genocide is intentional; mass-deaths due to introducing foreign diseases was inevitable.