First, "DC current" is redundant. It's alternating current and direct current.
Cars need a shitload more than 12V. I am not entirely sure why 12V was chosen for the battery - perhaps for the other things running on the car's electrical system. The 12V is stepped up to 100,000V by an inductor before the current is fed to the spark plug. The voltage needs to be that high so the current will arc ("jump" from one metal point to another).
Likewise, I'm not sure why 48V was chosen. It may just be the smallest amount needed to polarize the most common types of capsules. Anyone?
-Matt