Because Voyager is separated from Starfleet the Omega team cannot be brought in to deal with the problem, so she is forced to break the code of silence, and inform her officers of the Omega Directive which traditionally is only revealed to captains and admirals.
Because Voyager is separated from Starfleet, the Omega team (a specially trained group which would normally be tasked with handling situations involving Omega) cannot be brought in to deal with the problem. Janeway is thus forced to break the code of silence, and inform her officers of the Omega Directive which traditionally is only revealed to captains and admirals.