I think the biggest factor would be her lack of plot exposure -
unless her reveal stirs up some really serious shit in the process, it's hard to imagine a Tooru-traitor reveal being worth the years of build-up.
The operative word here being
willing.
Outside of some vague hints, we know absolutely nothing about how and why the traitor is operating the way they do.
For all we know, the traitor could someone who has been coerced into the role by AFO or one of his agents.
I agree that Kirishima is is an unlikely suspect - considering the amount of time that's been dedicated to establishing his backstory and motivation to be a hero.
Likewise I think we can safely dismiss Jirou, Iida, Todoroki and Bakugo on similar grounds.
To be honest, looking at it from a meta perspective - characters like Tsu and Mina strike me as ideal candidates to be the traitor; Prominent enough in the story for a reveal to leave an impact, whilst lacking a substantial enough backstory to place them beyond reasonable suspicion.
While Ochako also fits the bill reasonably well, her status as a shonen love interest and heroic foil to Toga - plus the fragments of backstory and motivation we've received throughout the series - makes the prospect of her being the traitor a whole lot less likely (unless Hori pulling some major Penn & Teller levels of narrative misdirection here, which would admittedly be pretty awesome

)