This is a historical question. Since 1919, Transylvania has been a part of the country of Romania. Before this time, it was a part of Hungary. So what made this territorial change possible?
It was not this woman:
Queen of Greater Romania in 1919.
It was these men In Alba Iulia:
This was confirmed as fact by the Paris Peace talks and the Treaty of Trianaon. So, why is Transylvania a part of Romania?
World War I. This happened between 1914 and 1918. An ally of Great Britain and France and Imperial Russia was Romania. There is of course the point that in Transylvania there is a high number of Romanians, more than half the population of Transylvania. The allies also did not want another war fought so soon after this war.
This is why Transylvania is a part of Romania after 1919.