white polished rice used to be produced manually, polished by hand. For that reason it's very expensive and only edible by those rich enough to afford it.

Then in the 17th century, rice polishing met industrial revolution (machineries) and can be produced in larger quantity and in a more consumer friendly price, although it's still limited to the urban area.

Long centuries habit is hard to break. Japanese peoples now can enjoy "luxury" used to be limited to the wealthiest. In that euphoria, they eat a lot of rice, but didn't ate as much of the other dishes, which unfortunately brings nutrition deficiency related diseases such as beriberi (they called it "Edo sickness" in the Edo period because it hits in Edo where many are wealthy enough to buy white rice)

Still white rice is not available to large part of the population and it's still a luxury food they dreamed of.

By the way, if you join the military, you can have white rice... which again *points up* the euphoria brings an unfortunate side effect.
Don't understand why they don't eat the vegetable and meat though. Probably the space in their stomach is too precious to be filled with anything they could have ate back at home.