Because it wasn't a battery issue, It was a loose connection or terminal corrosion issue that got resolved when the wires were removed from the old battery and connected to the new battery.
2 good answers. Poor connections may show full voltage, but can't pass full current. In a case like this, it's always good to clean the battery connections. Back when I used to drive much older cars, that was always one of the first things to try for weak cranking...
!2v systems require very good earth/ground return to work effectively. Keep all connections clean & tight.
All my 12V started equipment, has an additional earth wire from battery negative (-) to the engine crankcase. If lights or other systems are involved, I run an earth wire to them as well. Very rarely have a roblem.
