If as is required all aircraft monitor 121.5, then unless you are currently chatting with someone on a frequency obviously the best emergency frequency based on chance of being heard would be 121.5. Simply I think, many have found that boring and prefer some other channel that is not in the interest of safety.
What other channel? You get the ABC on the ADF, not a comms radio. You can listen to the ABC on ADF or you iPhone via Bluetooth and still have a listening watch on 121.5. Anyone cruising above 3,000 ft AGL who doesn't have the second radio tuned to 121.5 is guilty of poor airmanship.
Anyone cruising below 3,000ft is in a J3 cub and having a blast with the radio turned off.