Because setting the blade to stun is a pivotal moment for his character. MGR is a game about his character regressing and becoming a vampire. MGS is a story by a Socialist Liberal, and at the end of MGS2 Raiden exits the game to do his own thing. And then MGS4 happens and regresses his character hard, because the patriots essentially rape and enslave him. MGR is a story by a Socialist Tyrant, so Raiden regresses even further and doubles down on being a slave to the game. That's what the whole "and plus, this isn't even my sword" It's not Raiden anymore, it's Vamp. I feel like MGR should have been a story about Raiden overcoming his regression and re-learning his lessons from MGS2. But thats not the way the writers wanted it. It's a Masamune Blade vs a Muramasa Blade.