I suspect the reasons are (generously) keeping them out of the hands of people who would treat them poorly and perpetuate the stereotypes, or (less generously) ignorance and fear.
For example:
https://blog.dogsbite.org/2016/10/table-retrospective-level-...
https://www.dogsbite.org/dog-bite-statistics-studies-level-1...
"pit bull" refers to a dog breed that was optimized for its performance (=more aggressive and dangerous) in the "pit".
Speak about natural instincts... I answer that some people have zero clue what to do with a dog.
I say we put down all these golden retriever too !