Those 70 years include a period, 1946-1975, in which US maintained war in SE Asia, first supporting the French colonialists and then a series of Southvietnamese dictatorships. This was no doubt because of the primacy of internal political considerations over international rules of conduct, a objection you direct at current China. A more important matter is that China lives in Asia and so has more interest in maintaining peace in the area than US that live thousands of kilometres away.
I don't think it's correct to say that the US has had the primacy in Asia after WWII. Far from it, Asia had been hotly contested region during the Cold War, among multiple players including US, USSR, and China. There had been multiple hot wars and China was a big part in them (Korea War, Vietnam War, and other regional conflicts). It is only after the end of Cold War that the region has been mostly in peace. This peace has not been solely because that the US military primacy guaranteed it, but also because of the dissolution of USSR and the US-China rapprochement.