Why has the Japanese-American population decreased?

[dpmapper]

An older, highly secular population leading to low fertility (same reason Japan is losing population), plus a negligible amount of immigration (esp. compared with the other ethnicities you listed).  Seems pretty straightforward to me. 

[dpmapper]

There is plenty of Korean immigration, both student and otherwise.  Just check out any H-Mart. 

In addition, since the wave of Korean immigrants is much more recent than the Japanese, it's a younger population overall.