I've answered numerous times in this sub and I will try to keep my conduct in line with the rules like usual, but this subject has a lot deeper connection on many levels than many think. I'll try not to come across as biased. This question isn't necessarily a historic one, but more of a social question.

The United States had admitted and apologized several times in the past to their clear and obvious wrongdoings regarding Native Americans. Many people in different fields, whether that be scholarly, activism, social, political, or scientific, have recognized that what was done to the Native Americans was, and some argue still is, genocide. Check out this FAQ on the most active Native American subreddit, /r/IndianCountry. It definitively states why it was and is genocide.

However, many continue to refute that it was, particularly within the education system. The reasons for this are varied, but it comes down to a few simple ideas. During the time of the Trial of Tears, the people of the United States believed heavily in two doctrines: The Discovery Doctrine and Manifest Destiny. Therefore, encroaching settlers believed it was their God-given duty to take land from the Native Americans. These doctrines, while not necessarily believed today by any direct manner, have influenced the mentality of Americans.

Due to this influence, many are disinclined to call what happened a genocide because by mitigating that fact, it detaches them and their ancestors from any guilt or culpability. The education system of the U.S. is still largely built on this kind of foundation as is evident by the whitewashed history children are taught in current curriculum, such as the landing of the pilgrims and the Thanksgiving Day tradition. None of that is an accurate representation of what happened. And yet, they're still taught it. But that is just a minor example. The bigger picture is that society as a whole in the U.S. wants to refuse what happened because they do not want to share in the guilt. They rationalize it and play it off as if it was thousands of years ago. In reality, the U.S. government was still sterilizing Native American women up into the 1970s, which can constitute a form of genocide according to that FAQ and the provided links. A lot of it as well is that people just don't care or are ignorant. You cannot believe how many times I have encountered someone who didn't think Native Americans existed anymore. If they don't think we're here, why should they worry about what happened to us? I think you see my point...

Look at the global scale as well. Turkey and their government/culture is no where near as large as America's. They lack the capability to influence major populations on the other side of the country to things such as this. Many Turks will probably deny the genocide, but as it is made apparent by your own post, many people outside of Turkey don't see it that way. Compare it to the U.S. now. There are a good few nations who realize what the U.S. did was genocide. But how many are many a concerted effort to push that knowledge? The simple fact is that we're too marginalized in our own lands, let alone the world, to have an influence on other nations/people and we are largely subject to the interpretation that the U.S. wants to give for us.

Edit: some grammar

I think characterising the deaths of Armenians as somehow incidental from the deportation in 1915 is incorrect. During the deportations the Armenians were often attacked by armed bands, in some cases these bands were organised by local officials. When foreign observers complained about this, the centre government requested that the roads be cleared because it was untidy.

Further, massacres continued after the deportations of 1915. For example there were a number of massacres of Armenians in the areas they were deported to in Syria in mid-late 1916. The German Ambassador (Paul Wolff Metternich) complained about this, so the central government requested he be withdrawn. Taner Akcam suggests these subsequent matters were related to demographic policy (essentially there was too great a concentration of Armenians in those districts from the CUP perspective), but acknowledes there is no hard evidence to confirm that interpretation. It certainly doesn't fit with the usual narrative that the crimes against the Armenians were simply an unfortunate side effect of deportation.

The starvation of the Armenians was essentially deliberate too. The Turks had promised the Armenians that they would be somehow compensated for property seized when they arrived in the deportation zones, for example that this would be used to provide food etc. This did not occur. The government specifically blocked foreign aid to the Armenians in the deportation zones, and there were a number of examples of people who did provide aid to the deportees being punished for it.

Taner Akcam "The Young Turks Crime Against Humanity" - Akcam sourced most of the above from Ottoman archives.

http://www.armenian-genocide.org/1916-2.html - provides a more detailed timeline of the 1916 massacres.