Because there are more factors beyond the long-standing ideological project to spread the doctrine of NATO (hiding behind terms like "democracy" and "human rights") around the world.
For instance, the US has an enormous mutual economic interest with PRC, and recognizing ROC's claim to virtually all of mainland China (the PRC) would be enormously complicate that crucial material relationship. ROC and PRC are mutually exclusive concepts because they claim the same territory, and PRC is the far more valuable economic partner.
