because if the soul of the caster gets taken, then the biiju would freely take over the body. Its not like Kushina couldn't use RDS at that time since wasn't even a jinchuricki at that time. Kyuubi was already extracted so she was technically free.
I don't know why exactly, but it definitely has to do with their Biju's soul. The RDS needs a soul as a sacrifice, Jinchuriki's technically have two souls in them.
Maybe it will be explained later in the manga. Zzz
Maybe it's similar to a genjutsu being cast on its Jinchuurikki...the bijuu may instinctively disrupt the flow of chakra therefore disrupting the jutsu?!
I say, because a jinchuriki conrains 2 lives (binu + jinchuriki) so itd be like one life wasted... andit wouldnt be justified..even for death reaper seal...