Car batteries work through a chemical reaction and during cold weather the chemical reaction in the car battery slows down.
So that's a reason that cold weather makes the car batteries go low in the winter and makes it hard to start your car when your cars battery has been subjected to the cold.
Eventually if you leave the battery out in the cold weather in your vehicle for a few nights or so then the battery can drain completely and need recharged before it starts your vehicle up.
During extreme cold weather it's best to keep a trickle charger on your cars battery to keep a charge in the battery so you don't wake up to a dead car battery.
I installed a trickle charger permanently on my cars battery where I just plug the trickle charger in during extreme cold and let the battery get a charge overnight.
After that I never had a problem so far with car batteries going dead.