"Harvey, I'm Batman!"

Reactionary to the situation or to distract him maybe. It wouldn't have mattered. Harv or anyone else probably wouldn't believe him.

I wonder if any of the batman actors shouted that at Harvey Weinstein for lolz at one of those post Oscar parties ?..

Then again Keaton probably has never watched Batman Forever and even if he had on a family viewing probably wouldn't want to acknowledge it or even remember that line, Kilmer would make most sense but didn't Clooney work on some Harvey movies so its possible he made Harvey lol with it .. but I think Affleck would've been the most likely candidate as he's probably nerdy enough to do it and seems to have had like a mentor type friendship with him but hed only have about a year or 2 to do so before Harvey finally got nailed...

But in any case maybe none would risk it in case they thought Harvey had never seen BF so the joke would fall flat and then Harvey would think they were being a dick and not consider them for his latest Oscar winning masterpiece..

And I've clearly given this too much thought..

1) Desperation 2) Harvey forced him to reveal himself.

I love that moment. This movie is mostly a wobbly mess, but that moment is really great. I think it's great because it shows a quick, decisive action from Bruce that he cares more about the lives and welfare of others than he does about his own. That revelation would ruin him, but he does it anyway. He does it quickly, too. It barely requires two seconds for him to do it. That's a hero.

As to why...

(1) When he rises and tries to shout, it's before the general hubbub starts. By the time he's opened his mouth, the crowd has started up and it's ineffective. But a fraction of a second more and Bruce would have been successful.

(2) The chaos that sets in prevents Dent from hearing, but it prevents others from hearing, too. There is SO MUCH NOISE and ado that nobody hears or processes what Bruce says. Even people right next to him clearly aren't in a position to understand what he's saying.