Minor correction to your questions, but while the Ryukyu Kingdom was definitely independent, it also paid tribute to China and Japan. During the mid-to-late 1800s competition for influence over the Ryukyu Kingdom between China and Japan was rising, ultimately leading to Japan completely incorporating it as a territory.

After the United States forced Japan into unconditional surrender after the end of WWII, it allowed them to have control over what territories Japan could keep. Part of making the Ryukyu archipelago part of Japanese soil was undoubtedly so that the United States could keep a very strategic base in the Northwest Pacific, and prevent any influence from China/Taiwan whom could make some claims over their influence during the times of tribute.

The Okinawan people are absolutely not on good terms with the American presence that has caused countless incidents and mishaps over the decades since WWII, (helicopter crashes in civilian areas, abuse, even rapes) and there is constant debate about how much presence the Japanese government should really allow the American base to operate at.

Then this opens up another can of worms, what about the Okinawan/Ryukyu people specifically? Well, like many indigenous groups during imperialism, they have clearly had the short end of the stick since the late 1800's, and have been largely powerless to fight for any true Ryukyu independence, but still at least fight for representation within the Japanese government.

This is a very abridged answer, and the only source I have on hand is relating to the status of the Ryukyu Kingdom as a tribute state, but the amount of debate going on regarding Okinawa is still a large point of contention between the Japanese, Americans, and people of Okinawa today.

(B. Walker, Jordan. (2015). Archipelagic ambiguities: The demarcation of modern Japan, 1868-1879. Island Studies Journal. 10. 197-222.)