If you reduce it to 2s you're lowing BRD's already weak burst window. If you buff BRD damage to fit into the 2s window you run into potential problems of making their damage too strong outside of the silence window. Recup and Guard are already relatively strong tools and are even more potent on jobs that have additional mitigation like healers and certain tanks. The whole point of silence is to hard counter those things. RDM was lowered to 2s because it hit multiple targets, but lowering brds to 2 seconds would hurt their ability to contribute significantly and simply "raising their damage" would not compensate for nerfing or removing their silence.