DIAGNOSIS The Auto Power Link feature, which uses HDMI-CEC, is either disabled or not communicating properly with your TV.
FIX Enable Auto Power Link on the soundbar. Press the gear button on the remote, navigate to "General," then "Auto Power Link," and set it to ON. Now, when you turn off your TV, the soundbar should go into standby (display off) within a few seconds.
IF THAT DIDN'T WORK The problem is on the TV side. Go into your TV's external device manager settings (often called Anynet+ or HDMI-CEC) and ensure the feature is enabled. You may need to toggle it off and on.
HEADS UP In standby, the soundbar still draws a small amount of power to listen for the TV's wake-up signal. To cut all power, you'd have to unplug it, which means you lose the quick start convenience.