Because Permanent Stat debuffs from an attack have the potential to become 'a snowballing merry mess of murder', in the words of one of the GM of probably the best PvP oriented RP I've ever been part of. What it does do is make your target more vulnerable to being Hack attacked by your allies. If it carried over to your next turn, the snowballing from that attack would unbalance it. Your Weave and Strategy abilities are fine, but I've been in an RP with a permanent stat steal character before. It does get unbalanced, and it does annoy the other players involved because your character ends up OP.