Asking why the US never accepted ally status, implies that China offered it. I don't believe that has been the case.
China and the US cooperate when it is in the interests of both nations. Both are large enough that neither needs the other, the economies are so intertwined that a war between the two counties would have serious economic consequences, but unlike the US and UK, there is no long standing shared cultural heritage.
Opinion: I tend to characterize the relationship between the two countries more as respect, than friendship. The interests of the two nations diverge enough at times that ally status is probably not something that will ever happen.
The blame is 100% on the Chinese side.
Immediately after WWII, the US tried to make peace between CCP and KMT. This effort was genuine but was nevertheless feeble because the volatile situation was beyond the ability of then American leadership.
When China's civil war broke out, American attitude was something like this: if war is how you settle dispute, we will let you fight it out; after that we will resume business with whoever wins.
As soon as China's civil war began to settle, the US made a gesture of détente by staying John Leighton Stuard in Nanjing, hoping to open up communication with the CCP.
But Mao had decided to side with the Soviet. In order to clear Stalin's suspicion, Mao gave the US a cold shoulder and wrote an open letter to ridicule Stuart’s departure.
4.1 Why Mao favoured the Soviet is still open to speculation. Of all the colonial powers, the Russians in Manchuria were famous for lacking the sense of superiority, not only because the Russians lived side by side with the Chinese but also because because Russians were frequent targets of Chinese scams - as was witness by a famous writer Zhu Ziqing - and fooling Russians had become a staple of Chinese jokes. Back at that time, Mao as well as the entire Chinese population were suffering from inferiority complex.
Stalin was still suspecting China of buttering its bread on both sides. Mao sent troops to Korea in order to draw American blood and, in political jargon, made his bones.
Between 1978 and 1991, China and the US were de facto allies against the Soviet.
After 1991, China became number 2. From a gaming point of view, no body likes number 2: number 1 is directly threatened by number 2, and the rest of the world would side whoever becomes number 1. Number 2 has to tough it out: either overthrow number 1, or wait until number 1 implodes, or become number 1 himself.