It makes sense.
Not only is it easier to look in someone balls than someone's brain, the fact that it can cross the blood brain barrier is pretty fucking crazy.
It's not like someone said "we gotta find out if this can get inside testicles stat!" It's that between cancer and fertility issues balls are looked at pretty frequently, and we found micro plastics there.
The reason we found it in the brain. Is someone asked if it was possible and that's a big enough deal to go out and look.