Because there's nothing else visually interesting about that costume. The black bat would make it look even more generic than it does. The oval gives it some contrast. It's striking. It's simple and effective.
This isn't directed you, but I don't understand the hate for the oval. It's not like Batman goes around flaunting his chest yelling at criminals "HEY LOOK AT THIS BRIGHT YELLOW THING ON MY CHEST!!!!". And i think alot of people forget that Batman actually wears a CAPE that can COVER it when it needs to be covered. People act like the thing radiates yellow light or something. Stealthy or not, he's still a damn comic character and there will always be license taken with reality to make certain things work.