One of the big reasons Japanese players don't come over is because by the time they are able to be posted, they're 26, 27, 28. Not many players want to uproot their entire life and acclimate to a new culture at that point.
One HUGE reason that Japanese players didn't come over in the 60's, 70's, 80's is because Commissioner Frick suspended MLB dealings with the NPB because of dealings the Giants had with NPB players. Frick threatened lawsuits and imposed an unofficial ban on teams dealing with Japanese teams.
So it's not like they weren't coming over because they weren't deemed good enough. They were effectively black balled.
And I don't think you're going to find any Japanese players coming over in the 40's because of WWII and internment camps in the US.
Another argument I'd present is that Japan is 2 time WBC winners. They are good enough to beat loads of MLB talent. That level of play doesn't just appear one generation, it's been cultivated since baseball was introduced (and immediately accepted as part of their culture) in 1872.
Japan/Taiwan have combined for 27 Little League World Series titles. Japan/S Korea/Chinese Taipei have 6 Olympic Baseball medals. They know how to play at a high level at all ages.
Again, I'm not saying every NPB player is on par with MLB players or Negro League players, but the best of the best surely gets some consideration... just like the Negro League players. I don't see every starting first basemen from the Negro Leagues getting picked; only the best.
Here's a quick breakdown of barriers Japanese players faced coming over to America (not including the posting system):
1935 and earlier-No professional Japanese baseball league
1936-1950-Wartime and occupation
1951-1963-Korean War to the Murakami Affair
1964-1994-Murakami Affair and the De Facto Ban
1995-2000-Pitchers Break the De Facto Ban
2001-present-Position players arrive
I won't say that it's exactly the same as to the barriers faced by blacks, but it is a very similar plight (I can't think of a better word). And while blacks had the freedom to sign with teams once Jackie broke the color barrier, the Japanese set-up the posting system to make it harder for players to jump to America.
So my overarching theme, I guess, it that talent isn't why there's not a ton of Japanese players throughout MLB history, politics, war, and a pissed off commissioner is why.