I've been reading text for one of my history classes and it talks about how terribly the Indians were treated when the new settlers from Europe came over and how they basically just ravaged the land and brutally murdered everyone including women, children, and babies. It talks about raping, enslaving, starving, torturing, burning alive and all kinds of insane barbaric stuff that seems way more brutal than the Holocaust, though i'm not trying to discredit the atrocities of the Holocaust.
Well, a number of reasons, the most important probably simply that it happened longer ago, partly this just contributes to an 'out of sight, out of mind' attitude and partly because we(in a popular consciousness sense, not historians) just think of the 15th and 16th centuries as inherently violent, 'backwards' times, meaning atrocities committed then seem less shocking than ones committed by 'civilised' people in the 20th century. In addition to this of course, the Holocaust is in living memory, there are still survivors around to relay how awful it was, which isn't the case with the European colonisation.
Also, while enormities of the kind you describe certainly did occur, the interactions of Europeans with native Americans were slightly more complex than straight up conquest and slaughter. There were also treaties, alliances and coexistence in places, and its still an open question the extent to which the destruction of native peoples is attributable to deliberate violence or accidental spread of disease.
In contrast, there are no such ambiguities about the Holocaust, it's one group of people deliberately and industrially attempting to destroy another group of people.
Mainly though, I'd say a lot of people probably do view the European colonisation of the Americas as a cataclysm comparable to the Holocaust, but it happened much longer ago and over much longer period of time, and is just therefore less imprinted on the public consciousness.