A few months ago, Sony planned on releasing Killzone as a PS2 Classic, however the project was canceled before it released. Since then we’ve heard nothing about Killzone, but it seems the removal might have been part of a larger plan.
Earlier today, Amazon France listed Killzone Trilogy for preorder, which could be next franchise in the collection series. Along with listing the title, the game was listed with an October 24th release. While none of this is set in stone, it could explain what happened with the original Killzone.