It's the chemistry. If you've ever taken careful measurements of your car battery's voltage, you'll find it's 12.62 volts (not 12 volts). There are actually six cells forming that battery, and each cell has a voltage of about 2.1 volts. It's the chemistry (lead acid) that determines the voltage.

A Nickel based cell has a voltage of about 1.2 volts. Alkaline is about 1.5 volts. The old cells we used when I was a kid are now marketed as "heavy duty". They're less expensive than alkalines but are a waste of money. The chemistry there is carbon-zinc, and they are also 1.5 volts.