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.
TEST, don't just replace! Have the battery load tested, not just a simple volt test.
If you don't tighten the bolt on the cable, it becomes loose, corrodes and loses contact, as others posted. If you did tighten it, do you leave the cables on and float charge it during the off-season? Better to disconnect at least one and use the float charger, then clean the clamps/ends the next time it's needed.
