Extracted from the Warner Bros forums:
Dear owners of the PC version of Batman: Arkham Knight.
We want to apologize to you who are having performance issues with Batman: Arkham Knight on the PC. We take these issues very seriously and therefore decided to suspend sales while we work to resolve these issues to meet our quality standards. We highly value our customers and know that while there is a significant amount of players who are enjoying the game on the PC, we want to do everything we can to make the experience better for the players.
Thank you to the players who have given valuable feedback. We are continuously monitoring the topics posted in our community and in the Steam forums, and any problems encountered with our Customer Service (support.wbgames.com). If you purchased your copy of the game and are not satisfied with your experience, then we ask for your patience while these problems are resolved. If desired, you can request a refund in help.steampowered.com (Steam reimbursement policies can be found here: http://store.steampowered.com/steam_refunds) or the retail location where you purchased the game.
Fans of the series Batman: Arkham have continually supported the franchise to its current height of success, we want to thank everyone for their patience as we work to deliver an updated version of Batman: Arkham Knight on the PC so you can enjoy the final chapter of series as it was meant to be played.