Why is the MSX almost non existant in the U.S. ?

My speculation from a historical point of view In the 70s and 80s some people/policies were anti Japanese. It would have been in their interest to sell and grow domestic products rather than imports. Same in the UK (see the Sinclair interview on MSX).
Due to the success of Japanese electronics and cars, it appeared as though failings of domestic companies were due to Japanese products in the market.

I would say politics played an important role.

e.g this is from 1987

https://www.nytimes.com/1987/04/18/business/president-impose...

Not in the early 80's and the average buyer wouldn't know/care.
.
Early Commodore machines were 100% built in Japan, and when they moved production elsewhere many parts (FDD drives, TV modulators, Keyboards, PCBs) were still made in Japan.

There's only ONE reason: price of the machines and who came first to the market.

Early MSX1 machines in Japan won a major market share because there where no competitors in the low-price segment, people wanted cheap game machines or cheap hobby computers and there where none on the market. Then came the Famicom and NEC 88/98 machines and the war was lost.
In Europe the Spectrum won initially the UK market because is was the cheapest decent machine one could buy.
In the US the market was saturated by many manufacturers, who by 1984 already had established market share, sales infrastructure and local hardware/software support. Apple and Atari were selling the same machines from 1977 (Apple II) and 1979 (Atari 400)!
Microsoft wasn't involved into MSX marketing, they were 100% committed to the (IBM) PC market as it was their golden goose.
And finally Japanese manufacturers were dumb: They did sell machines but they themselves didn't know their target customer. They should have sold cheap game machines but for some reason only Nintendo figured that.

We can imagine the alternate universe where a Zemmix-style MSX dedicated game machine competes with NES and Colecovision, but I don't know how it could have competed with later NES titles that were made once NES memory mappers were widespread and developers understood how to really use NES PPU

Aside: did anyone ever try to get a NES PPU working in an MSX? At least the specs seem a little bit like an MSX 1 VDP but with some nice additions

I don't mean to derail, but I wonder the same thing about the UK.
But for both reasons (USA as well), it was probably marketing, and later that the MSX(2) couldn't compete anymore against platforms like the Amiga, SNES, MD or TurboGrafx.
I do still wonder why MSX never set foot as strongly in the UK. You'd think it would be a natural evolution from being ZX Spectrum afficionado's, don't you think?