Why is Superman wearing his black suit at the end of Superman's black suit, pulled right from his resurrection after dying in battle with Doosmday in the comics. Though this is an integral element of Superman's return to life, it functions in a much more symbolic role in Snyder's story.
In the comics, Superman's black suit served as an energy conduit to get Kal-El back to full strength after his return to life. In Snyder's of Superman's resurrection, he has no such need to recharge his batteries, as the Heroes Park battle with the Justice League clearly shows him at full strength, albeit confused and disoriented after having just returned from death. When Superman s the League in battle against Steppenwolf, though, he's clad in a brand new black and silver suit from the Kryptonian scout ship.
Clark looks to have adopted this is as his new primary suit by the end when he rips open his shirt to reveal the silver S-shield underneath. Logistically speaking, Clark needed a new suit anyway; his previous red-and-blue suit had a gigantic hole in the chest from being stabbed by Doomsday. As for why Superman continues to wear his black suit after the fight with Steppenwolf is over, this goes back to Snyder's plans for Superman in the five-movie arc he had mapped out, particularly how Clark finds his way back to the original red-and-blue costume.
Snyder has spoken of Superman's story in that signify it as a journey, his death in Superman was to learn something at every step, with the plan being that his return to the red-and-blue suit would mark the completion of his journey.
Additionally, Snyder also described the black suit as "a great time delineator" to visually indicate where Superman was on the road he'd planned out. In a nutshell, the iconic Superman shirt rip at the end of the film features the black suit due to the Snyder Cut marking the mid-point of Kal-El's story. With the film bearing the title Zack Snyder's Justice League, it's not surprising that the director would want Superman's portrayal to be consistent with where he wanted him to be at this point in the storyline.
The question now is, will the rest of Snyder's planned story ever be realized? At the moment, there a no known plans for any further films in Snyder's DCEU. With that said, with Superman in the black suit as Snyder had wanted, and the cliffhanger ending of Zack Snyder's Justice League, the cards are all on the table now for the story to be finished if WarnerMedia wishes it so.
