https://www.nexongames.co.kr/bbs/board.php?bo_table=media_event_en&wr_id=421
>“The launch of the Steam version is an important opportunity to showcase the game's appeal to a wider global audience and further expand the IP,” said Kim Yong-ha, Executive PD of Nexon Games. “We will do our best to deliver the immersive story and strategic battle fun of ‘Blue Archive’ on PC.”
There's your answer for why they released the port on Steam instead of making a standalone client or releasing it on the Nexon launcher.
