For starters they wouldn’t want to be seen as oppressors and foreign dominators in the face of a people seeking self determination after they had already crushed an insurgency.
Secondly the US was losing faith rapidly in the KMT while it was falling and it was not guaranteed that the communists couldn’t reach the islands, there was an expectation that the island would fall. An independent Taiwan might even be considered one of the only avenues to keep the island out of communist hands at that point.
Given how supportive Taiwan was for Imperial Japan compared to Korea (there's a reason why the Japanese felt comfortable conscripting more Taiwanese per capita compared to Koreans), nobody is going to care for what the Taiwanese want, especially when Chiang can count on the China Lobby in DC to lobby for him on an island nobody cares about.