Partly due to what it cost the US public. Free trade with smaller countries hurts less than one-sided free trade with a protectionist mercantilist country 4x your population. (one-sided due to required joint ventures, unilateral bans on US social media, currency manipulation, among others)

> and singlehandedly denied it UN representation for 20+ years.

To be pedantic, the US didn't deny China representation for 20+ years, just the CCP. The ROC was a founding member of the UN and permanent Security Council member. US just opposed the CCP replacing it until 1971. Given the instability of CCP regime during that time - famine, Cultural Revolution - that wasn't an unreasonable position.

> similar to those it has with nearly every other country.

Except the Communist bloc countries, USSR, Cuba, etc. Extending MFN to China while they were still Communist was unprecedented.