Why did Britain fight so hard to prevent Irish independence? And why is theren a continuous struggle overNorthern Ireland?
Why does the Republic of India insist on hanging onto Kashmir, refusing a plebacitie?
Why would Sri Lanka not allow an independent Tamil state?
It is really very normal for states to refuse to give up territory. The Lhasa government claimed independence after the Revolution of 1911-12, but went back on it later.
The current Dalai Lama was selected with the partici[ation and approval of the Central Government, which was not however in clear control of even the Han core of China.
What happened in 1949-50 was a reunification of China. No officila body recognised TIbet as an independent state, then or since.