Those were under Chinese sphere since Tang dynasty. But not a formal sovereign subject. This relatively weak bound was fine at the old time, since the farther outsphere does not have the influence to encroach to the Chinese sphere.
In 19 & 20 century, things changed. Those strategic areas have to be under China's control for national security, which was under siege from Russia (the most expansionism nation on earth).
Like USSR inherited Russia's imperial territory, and India inherited great Britain's, CCP conveniently inherited Qing empire's as well (indirectly through KMT, the nine dashed line was a KMT invention...).
In 19 & 20 century, things changed. Those strategic areas have to be under China's control for national security, which was under siege from Russia (the most expansionism nation on earth).
Like USSR inherited Russia's imperial territory, and India inherited great Britain's, CCP conveniently inherited Qing empire's as well (indirectly through KMT, the nine dashed line was a KMT invention...).