Probably the easiest explanation would indeed be that Cartwright and/or Chang, being members of the conspiracy, intentionally diverted security away from the conference so as to make it easier to carry out their plans (and also to blame whatever happened next, on Kirk).
As for the out-of-universe explanation: It would have distracted from the pacing of the film.
As for the out-of-universe explanation: It would have distracted from the pacing of the film.