http://tvline.com/2012/06/19/comic-con-community-castle-2012/

Turning to Castle, there’s no official explanation regarding its Comic-Con disappearing act, but Castle scribe Terri Edda Miller tweeted late Monday that “the show was not invited,” adding, “[ABC has] news shows [to] put forward now.”
One possible bright spot for Castle fans: We hear leading man Nathan Fillion is slated to make an appearance at the convention, although the what/when/where of it all is still being worked out.