In 1924,the ANSI - American National Standards Institute, USA, proposed a simple system to designate cell sizes, based on the letters of the alphabet, starting with the smallest cell in volume and going in ascending order, A, B, C, etc.
B batteries never really caught on commercially, at least on the consumer end of things. B batteries were primarily used in devices that used vacuum tubes which is outdated technology. A size batteries are hard to find as well, because they were primarily used in really old laptop batteries.
The ones that were most popular just ended up being the AA, AAA, C, and D. Most batteries (AA, AAA, C, D) all carry the same voltage, 1.5 volts, but they differ in their capacities, their physical size.