remember that one chapter where they got caught by Ito's organization and Kyousuke got heavily drugged?
Yeah but that happened long ago before he lost his memory
The trauma of what the organization did caused his memory to be lost. That's why.
this kept on being asked. just read this.
http://www.mangago.zone/home/mangatopic/2165855/
my longer version of explanation http://www.mangago.zone/home/mangatopic/2166142/
Did I... Miss something? How did he lost his memory?