Since you put it that way, here's my brutal explanation. Because they weren't able to defend their own freedom when it mattered.
They had their chance. Tibet and Mongolia both broke away when the Qing collapsed in 1911. Taiwan became de facto independent due to special circumstances in 1949. Why didn't Mongolia get reconquered? Why is Taiwan still a thorn in China's side until today?
Mongolia doesn't have a large or advanced military, but it was able to hitch itself up at once with big brother the Soviet Union. Taiwan also has the US to protect it all this time. Tibet when it was independent? Nothing. Its army when the PLA invaded was something from out of the Middle Ages. What's more, they failed to make "powerful friends" who could have applied pressure on an agressor WHEN IT MATTERED!
50 years after the fact, just when China is beginning to treat Tibet (and Xinjiang, and itself) right, we now have all these bleeding hearts wailing about how every ethnic group in it should be carved out into a separate nation. Where were they when the Chinese were dying in droves during the famines of the 1950s? (Stupidly self inflicted true...) Where were they during the chaos of the Cultural Revolution? (Now THAT should have been enough justification for intervention IMO.)
Most stuff that posters here are quoting date back to at the very latest, the late 1980s. Things have changed since then. Maybe not much by Western democratic standards, but they have.