We would later learn that this was among the changes made to Justice League following Zack Snyder’s departure and Joss Whedon taking over helming duties.
In fact, Snyder planned on having Superman go on a journey across subsequent movies to earn back the blue suit.
As for why this black suit wasn’t included in Justice League’s theatrical cut, costume designer Michael Wilkinson, said back in 2018 that as the tone of the movie developed, the filmmakers felt that it was “more appropriate” to have Superman wear his classic red and blue suit upon officially returning to action.