I suppose they need a case where they can establish the involvement of one or more specific GB members, though they can also do what the royal commission in Australia did, which was to ask questions regarding policy in order to determine if protocol had been followed by elders in the cases they were investigating. But that commission is not the same as a criminal investigation.
It might also be that, lacking a specific link to a specific case, prosecutors in the USA might be hesitant to involve the leaders of a church in an investigation in an attempt to demonstrate a broader problem. I am sure that many other church leaders would enjoy watching GB members squirm, but they would also realize that if the WTS can be treated like this, so can they. They would protest the treatment of the GB on general principle.
I do think that the USA should have investigations like the ones in the UK and Australia. But, again, there would likely be a lot of pushback against the idea, since the USA is still very religious.