Despite Kancolle is now no longer enforcing lotteries for registration, occasionally the game server(s) would be full and would therefore stop accepting new player(s). Usually that would be addressed within the same week whenever such an event occurs. You may wish to read the game's official twitter page for further details, or the information above should there be any (rough) translated official tweets. Occasional system maintenance of the game will also stop new players from registering new account(s).