I'm probably misremembering a lot but, right now, the inmates at Guantanamo are considered something like Prisoners of War... but not. Most of/All of them have not stood trial.
Moving them to the USA mainland and trying to dump them into the USA civilian legal system to "stand trial" would result in them being let off on any number of technicalities such as "being tortured for many years" and "being detained with little or no evidence". And we can't have that, now can we?
and trying to dump them into the USA civilian legal system to "stand trial"
No, they'd still be kept in indefinite detention without trial, but on American soil. That's the reason that several progressives voted to block the move, because it would've established further precedent to expand those conditions into the US. It wouldn't have given them a fair trial (by US standards) or stopped torture, it would've just allowed them to be held without trial and tortured on US soil.