The character is supposed to blend in, in order to leap from the shadows to stop the bad guys, so wearing anything other than black simply wouldn’t work.
One is that, Joel Schumacher movies notwithstanding, the cinematic Batman has always been designed in a world where pretty much everything around the character is also black.
While we don’t know for certain what the answer to Jordan Vogt-Roberts Twitter question really is, there are a couple of points that stand out.
Additionally, you have the issue that the blue and grey costume may still have some negative connotations attached to it because its most famous use was probably during a version of the character the films do not want to remind people of, the 1960s TV show.