They were specifically targeted over the course of United States history, why was it never ruled genocide by the international community? Sure most of it happened before the term was coined, but so did the Armenian genocide.
Alright, alright, alright. Let's get down to business.
Here are some previous threads about this question I've answered in the past. Disclaimer: the older ones are not as polished as my more recent answers.
Why is the Armenian Genocide a genocide while the Trail of Tears isn't?
Why is the European colonisation of North America seen as so morally wrong?
So this question really has different aspects to it. In a number of circles, whether they be academic, political, or social, the atrocities committed by the United States government are counted as genocide (multiple genocides, in reality). However, it is also denied by circles within those fields as well.
Speaking of academia, the charge of genocide has been highly debated for many, many years. In the 60s and 70s, we start to see the beginnings of a general shift of opinions on this subject to accepting the actions as constituting genocide. However, it is by the late 80s and 90s that acceptance really takes hold. Works by numerous experts emerged. David E. Stannard, author of American Holocaust (1992) and Ward Churchill, author of numerous publications such as A Little Matter of Genocide (1997) became big proponents of this claim and presented well researched and documented evidence to back it up. Vine Deloria, Jr., author of Custer Died For Your Sins (1969), also commented on it many times in his works.
But opposition to this was encountered. The narrative of what happened to the Indigenous peoples of the Americas has hosted many fallacies, misconceptions, denials, and outright lies. The obscuring of truth behind this matter can happen for several reasons. Stannard (1992) puts it plainly when he says in American Holocaust:
Denial of massive death counts is common—and even readily understandable, if contemptible—among those whose forefathers were perpetrators of the genocide. Such denials have at least two motives: first, protection of the moral reputations of those people and that country responsible for genocidal activity . . . and second, on occasion, the desire to continue carrying out virulent racist assaults upon those who were the victims of the genocide in question (p. 152).
Thus, debate around this abounds, both on the professional level and within the local classrooms of public education centers. James Fenelon (2003) writes in an issue of American Indian Quarterly:
The "academy" of scholars in United States institutions of higher education generally do not like hearing about genocide in the Americas, especially if it implies or states that this country willingly participated in and benefited from genocidal policies.
Whether online or in the classroom, denial still exists. Here is an example (Lewy, 2004). Furthermore, Brendan C. Lindsay, author of Murder State: California's Native American Genocide (2012) notes that the California State Department of Education denies genocide being committed and sponsored by the state (Trafzer, 2013). Textbooks utilized by the public education system in certain states have a history of greatly downplay any mention of the atrocities committed, if they're mentioned at all (DelFattore, 1992, p. 155).
Politically, there is investment for the U.S. governments, state and federal, to largely deny the genocides. This is because of two reasons: the federal Indian policy and international condemnation. The former is more important than the latter because there is little chance of a retrospective charge of genocide being levied against the United States. The first refers to the responsibility that the United States has charged itself with regarding American Indians and tribes - that they are obligated to help them (Pevar, 2012). While the United States claims "plenary power" and can ultimately ignore or terminate the tribes, an acceptance of genocide at a governmental, particularly national, level would open up the government to lawsuits and demands by the tribes.
Socially, many of the same reasons Stannard (2012) pointed out are true. People simply do not want to acknowledge it because it looks bad and could carry implications that they don't want seemingly imposed upon them. Many would rather believe that we, Indians, "lost the war," "didn't do anything with the land," "died off from disease," or "were so backward and lowly before the coming of the white man." And it is these very ideas that breed what we have today: denial.
But again, we have started to see a shift. The younger generation of today and the field of academia seem to be accepting the genocide more than those of the past. As more research is done on this subject, the truth becomes more clear: genocide(s) did occur.
References
Churchill, W. (1997). A Little Matter of Genocide (1st ed.). San Francisco: City Lights Books.
Deloria, V. (1969). Custer died for your sins: An Indian manifesto. University of Oklahoma Press.
Deloria, V. & Wilkins, D. (1999). Tribes, treaties, and constitutional tribulations (1st ed., p. 4). Austin: University of Texas Press.
Lindsay, B.C. (2012) Murder State: California's Native American Genocide, 1846-1873. Lincoln: University of Nebraska Press.
Fenelon, J. V. (2003). Indians teaching about indigenous: How and why the academy discriminates. American Indian Quarterly, 27(1), 177-188.
Lewy, G. (2004). Were American Indians the Victims of Genocide?. Historynewsnetwork.org.
Pevar, S. (2012). The Rights of Indians and Tribes (4th ed.). New York: Oxford University Press.
Trafzer, C. E. (2013). Book review: Murder state: California's Native American Genocide, 1846-1873. Journal of American Studies, 47(4), 2.
Stannard, D. (1992). American holocaust (1st ed.). New York: Oxford University Press.
Edit: Grammatical corrections.
https://www.reddit.com/r/AskHistorians/comments/5pyxx8/genocide_of_native_americans/
somewhat relevant earlier answer
Under the UN definition of genocide, the way Indigenous peoples in the US and Canada can be recognized as genocide, although as I said it is a term with political meanings and baggage.
For one, much of it happened over a significantly longer time period than more "traditional" genocides like the Holocaust or the Armenian Genocide, in addition to its existence mostly in the past. Also, much of that genocide is what is often qualified as "cultural" genocide, in that it often wasn't attempted direct eradication of a living people but the destruction of their culture and traditions. An example of this genocide in actions are the Residential Schools in the US and Canada, where indigenous children were torn away from their traditions and often beaten for things as simple as speaking their language. /u/snapshot52 , a user whose response has been mentioned in this thread, talks about this on an episode of the AskHistorians Podcast.
Personally, while I don't like the term "cultural genocide" because that is genocide, I think its otherwise an excellent podcast on the topic. But yes, the long and the short is that under the "official" UN definition, and in the opinion of many scholars, the treatment of the indigenous peoples of the US and Canada can be called genocide.
With that being said, in terms of more physical violent actions, that's a more mixed question, particularly since the debate on the significance of disease to the decline of Indigenous populations is still ongoing, as is the nature of intent as it pertains to this topic.