Besides Rome doesn't really exist anymore; its influence is ongoing, but it is an ancient civilization and it is common human practice to refrain from passing moral judgment on ancient civilizations because morality is subject to change. The conquest of the Americas is much more recent and many people on either side of the story still live with the direct consequences. At the end, time is probably why.

Walter Scheidel, probably the foremost social historian of Rome today, once memorably compared Rome to ISIS--the main difference, as he saw it, is that ISIS has yet to throw men to be torn apart by beasts. Other historians have compared the effect of Roman slavery on surrounding societies to the Trans-Atlantic slave trade's on west African society. So people hardly ignore these issues.

That being said, there are several things about Roman imperialism that make it a much less "live" issue than modern slavery and colonialism. The first, obviously, is that it was a very long time ago. These wounds have healed, and unlike, say, the British response to the Mau Mau uprising we can't speak to the victims of Roman conquest. We have very little in the way of testimony from those on the business end of the Roman war machine, which is in stark contrast to more modern equivalents.

But there is also a difference in the end result of conquest. The Romans displayed a sort of brutality that would make even the most hard-hearted colonial governor blanch but, on the other hand, their conquests were not "continuously renewed", so to speak. If you look at, say, the US conquest of the West, the status of Native Americans as a conquered peoples in a sense did not really go away and was frequently enforced through government measures (such as deportations). The Romans on the other hand did not really have the capability or the inclination to create a colonial system, and so in many respects the hand of the administration would be quite light. In fact, in Macedonia and Greece the Romans boasted that the population's tax burdens decreased after conquest. Which meant that after a few generations there were not really the barriers to mainstream society that were erected in colonial systems. Which is not to say that the Romans were any better than the Americans or British, but their conquest systems were less self perpetuating.

Because we don't. There are a lot of historians who work on "the evils" (circumscription) that Rome did and Roman conquest brought. And you won't see many historians who idolize the Roman empire. But the idolization was at least real until the Second World War. The cruelty of Rome was seen rather uncritical. Why? Because Rome was the "founding father" of Europe, and the European tradition owes a lot to the Romans.

Real history often enough doesn't have anything to do with popular culture and their perception of history. So I think your question should be why in popular culture Rome is seen much more uncritical than e.g. the mass murder of Native Americans.

edit: ANd the answer to that question would probably be because we can see the descendants of the victims and the societies this brought into existence. Slaves in Rome weren't a cohesive group, and the mass murder/ genocide of celts during the conquest of Gaul doesn't have any people who identify with them, as even the French are more or less in the tradition of Rome, and not in the tradition of Celtic Gaul.