*WE* may be able to spot Wonder Woman in civies and calling herself Diana Prince, but I'd be willing to bet the vast majority of people who go to see that film have never even heard of "Diana Prince". Assuming they even go that root, it's likely they'll keep any reveal for the last reel...or even a post credits scene.
As others have noted, they are promoting the character as Wonder Woman already. The audience isn't going to be surprised, nor is that necessarily a perquisite to a good film.
For example: To go back to TDKR, Anne Hathaway's character is never called "Catwoman," but from pre-release publicity even people who have never picked up a comic book knew from the first second on screen who she was supposed to be. And did that "ruin" the movie? Of course not.