Thinking back, it was kinda strange mikasa was the only one to act. I guess the others were looking for more definitive proof, but once mikasa did her initial attack and Reiner did his proclamation afterward, everyone else - being on high alert - should have been right on them trying to kill them.
She seemed to be the only scout to overhear the initial reveal as they were walking away, partially because she was the closest, partially because it was so casual, and also because of course she's so protective of Eren that she'd be hyper-sensitive to the two suspected to be in cahoots with Annie to be pulling Eren to the side for any reason.
I think it's also because they had no back up plan that we know of and fighting the Titans out in the open, on the wall of all places, was the last place they'd strategically would want to engage them. So once they caught wind of what was happening, everyone seemed to be waiting (and hoping, I assume) to see if Eren would deflect the situation or de-escalate things rather then being ready to throw down then and there. Even Bert seemed to be caught totally off-guard by Reiner's decision to talk to Eren right there on the wall and Reiner didn't seem to think about what it would mean if Eren were to refuse. lol
Mikasa was the only one thinking on her feet in the moment, probably already analyzing the situation if things head south and probably just due to her desire to protect Eren, even if it meant throwing herself in the middle of a very bad situation she might not be able to walk away from.
My favorite part about that moment is that Mikasa's attack was clearly a reactionary, spontaneous one but it seems she at least had the plan to lop off their heads. Since Reiner was able to stop the blade with his hands, which Mikasa lost, she must have realized the blades are too dull and fragile to attempt that again which is why it looks like she decides to just slice Bertholdt's neck since he also put his hands up but she doesn't even cut them. If they were humans that should have been enough, but unfortunately it wasn't. It's also interesting that knowing she only has one blade and one shot to probably only take out one of them, she decide to go after the Colossal Titan, even attempting to deliver the finishing blow while he's on the ground rather than attempt to kill Reiner who isn't even at least critically injured like Bertholdt is.
I think Eren was the only one in the situation that could have potentially helped Mikasa kill those 2 before they could transform, but he was still in shock too much to be of any use, even when Mikasa's own life was in jeopardy, which is kind of bullshit.