It is in the strategic interests of the United States that no potentially hostile power become a trans-continental Empire. A Europe dominated by the Third Reich would have had enormous resources at its disposal. Assuming such a victory in this war it seems inevitable that Germany would have metastasized to North Africa and the Middle East as was their long term intention. Likewise an Imperial Japan that holds China, Indonesia, South East Asia, possibly India, and large swathes of the Pacific under its sway or thrall is an Empire that can shut the doors of trade to the United States and begin to infringe upon it's influence and reach.
Long term threats and the risks that you face when large tracts of the globe fall under the dominion of an ideologically incompatible or politically hostile power.
Trade does not flow freely, political good will does not flow freely. American non-interventionism was a policy dictated by our weakness in the very early stages of the 19th Century. Almost as soon as we gained a capably funded fleet arm we began defending our interests and stakes abroad, becoming involved in policing and fighting for a free slot in Asia by the middle of that century.
With the strength to intervene and prevent either a Nazi or Imperial Japanese dominated Asia and Europe it would have been ludicrous to allow that to come to pass uncontested. The consequences for American commerce and our political security would have been dire.
And of course the moral component of not allowing billions of human beings to be consigned to death, slavery, and oppression. But I've tried to avoid this as modern non-interventionists seem to take perverse pleasure in flaunting how little this matters to them.