protection

At that time，Japan was divided into several states as a unit of administrative.
Establishment of this division was era of dynasty.

And samurai regime period,there was lords who was appointed by shogun there.
They were called "Shugo".　(守護)

Literal translation of "Shugo" is "protection".


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Incidentally，this several states.
It’s called "Kuni". (国)
For example,”Hizen-no-kuni”，”Suō-no-kuni” etc. (肥前の国)（周防の国）　
　
Literal translation of "Kuni" is ”Country”.

State is called "Shū" at Japan. (州)

State some the country gathered.　
Although its popular name is not constant，Japanese at that time called it as shū.
　
For example，”Kan hasshū”. (関八州)
　
Literal translation of Kan hasshū is eight states of Kanto.
Kanto is Kanto region Tokyo exists.
At that time, the country of eight was present in the Kanto region.

"The Kyushū island" of the western edge of Japan. (九州島)　

Literal translation of ”Kyushū” is ”Nine states”.
Of course the country of nine was present in the Kyushū island.
　
By the way，they do not match the current administrative division.
Therefore we do not use currently calling of Kan hasshū
But Kyūshū remains as region name, including Okinawa.　


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Yes，all is this.
To tell the truth，this is not the case.

"Shikoku island" of southwest of Japan. (四国島)
This name is also currently remaining.　
　
Literal translation of "Shikoku" is "Four countries".　
At that time，four countries were present in Shikoku.
And now, four administrative divisions are present.
　
In some cases it matches like this.

Maybe there isn't a clear rule at there.

Ease of vocalization as Japanese.
This reason also might was important.