Probably because WB wants at least 2 DC Films a year (2016, 2017, 2019) rather than just one (2018). Since Flash got delayed, SS2 is starting from ground zero with Gunn, and Reeves just finished the script for The Batman none of them would have likely been able to make the 2020 dates WB set, and they're the furthers along, so moving WW84 seemed like the logical choice to guarantee a 2nd DC film in 2020.