Why didn't the hero association use Hitoshi the guy with the brainwashing quirk to make AFO leak information? Why they didn't use him which such a strong quirk? Did it got explained in the anime or manga, or did the author just forgot he even existed?
Since there isn't any objective answer to this, we can only speculate on the matter. Probably the obvious and simple reason is that Shinso isn't a fully fledged hero and exposing a kid to AFO interrogation isn't quite the most ethical thing to do and also AFO probably has barriers and others countermeasures to prevent brainwashing or mind reading.