Why didn't "Reunion 4" let Captain Surprise put on infinite gloves and snap his fingers?

Because there was no plan to destroy Thanos from the beginning.

Captain Surprise's task, like that of a spider, is to deliver the gloves to Ant Man's car, and then return the gems from the quantum field to the timeline.

Unexpectedly, at the last minute, Thanos threw a weapon in the distance, destroyed the car, and Captain Surprise was blown up. At this time, Thanos put on gloves. Although the captain of the surprise attack showed stronger strength than Thanos, Thanos used the infinite gem badly, so he took out the power gem and released the captain of the surprise attack with his bare hands.

At this time, the only one closest to Thanos was Iron Man. Doctor Strange reminded Iron Man to stretch out a finger, so we knew it was the only victory.

Of course, we all think that Doctor Strange ignored Captain Surprise. For example, he first made a plan to tell The Avengers to keep Captain Surprise, and then Captain Surprise snapped his fingers twice to save lives and destroy Thanos at the same time.

But this involves Doctor Strange's view of time. According to what he said in the last battle, even if Iron Man starts to refer to this plan, he can't disclose it, because once he says it, it won't happen.

According to our understanding of Iron Man, even if he knew he was going to die, he would snap his fingers, so why not say it?

Therefore, according to Doctor Strange's theory, the problem is not Iron Man, but the paradox of time. Just like our fortune-telling theory, once the secret is revealed, it will change.

So it's doomed that Captain Surprise can't be the savior, because even if Doctor Strange reminds Iron Man to keep Captain Surprise, the secret will change because he tells the plan, which means there may be no mice trapped in Ant Man's car.

what can I say?

Personally, I don't think there are any obvious loopholes in "Reunion 4". To say sorry, it may be that when Captain Surprise got the infinite gloves, no one had an idea to remind her that she could snap her fingers and destroy Thanos.