Great question. My opinion is they were either duds or once they were caught, un-exploded ones are easier to trace.
I’m not up on how mines work but I thought they exploded when a ship hits them. I didn’t know they attached to the ship and then exploded.
There are many options to mines, contact, pressure, magnetic anomaly, some even have a torpedo in them (against subs mainly) that are triggered by sound calibrated to the type of "noise" a sub makes as it goes by, or magnetic disturbance and of course timed. Limpets are almost always "timed"..
