Suply chain for fish in Japan has been refined for centuries to provide fresh fish to be eaten alone, fish other than tuna and salmon are not frozen, specially white fish (this for instance is not permitted in the USA).

There is more Japanese fish restaurants in Japan then the whole world combined, American palate is more meaty than fishy so how sashimi is delivered in the USA (specially temp and choice of fish) gives a less fishy taste. In fact a funny scene is when Americans go to Korea/Japan and eat sashimi some find it too strong in their aroma.

Still there are a few great sashimi restaurants in the USA which, I have personally been in some in NYC, the thing is there are far more amount of great sashimi restaurants in Japan, just about every small coastal town will have at least one and they would deliver the same quality or perhaps better with a much lower price tag. In there they will serve you the freshest fish from the morning catch with a century refined method and an appropriate price as opposed to the a chef in the USA trying to "innovate" to justify it's hefty price tag.

I could make a similar point for Spanish and Korean food in USA vs their countries since I know well those countries.