Hello there! You might be interested in this excellent answer by u/Plowbeast on this topic.

They considered it, but following WW2, the biggest incident that happened at the time in that area was the Korean War. North Korea invaded the South and almost took the country before the UN moved in and absolutely annihilated North Korean military, pushing them almost all the way into China.

China had to choose between helping out their allies in North Korea and stopping a potential push by UN troops directly into China, or to invade Taiwan and reclaim the last bit of China.

They chose to get involved in the Korean War.

Likewise, the US was initially pretty set on letting Taiwan fall, but after the Korean War began, the US put more emphasis on protecting Taiwan, and thus made it much more difficult for Communist China to invade.