Like I said previously. It probably has something to do with the tailed beasts used being "DEAD" inside the statue. During the Minato flashbacks they said they could just defeat and "kill" Kurama, but it would only keep him away for a limited amount of time until his chakra reformed or whatever.
Otherwise Kishi could've easily brought back previous Jinchuuriki for the 8 and 9 tailed beasts and had Edo Tensei'd versions of them, too. 8 and 9 are currently alive and in live hosts. So they can't be tethered.