My personal tinfoil hat take is that Xeon Phi was made to make China waste top dollar on their supercomputers going with a less useful accelerator made by a US company.
Once China figured that out, the way they'd implement not falling for the same trick twice is simply never using US tech again. Which would limit other opportunities.
It's also at least plausible that they'd probably have people trained to think like Americans who might think of the same thing in advance.
Yes US government has forbidden the export of Xeon and Xeon Phi to China's supercomputing centers. It prompted the construction of Sunway TaihuLight, which uses Chinese own CPUs.
The answer is rather simple. Xeon Phi wasn't as good as the GPU counterpart.