Have you ever wondered why your computer’s hard drive is labeled (C:)? Since this is the primary hard drive, (A:) would be much more obvious and a C seems a bit arbitrary, especially since under normal conditions you won’t find either an (A:) drive or (B:) on your computer. With a newly connected storage medium, the computer continues the alphabetical sequence: if you connect a USB key, for example, the letter (D:) will probably be assigned to it if the letter is not already occupied by another drive; in the latter case, the stick would receive the designation (E:).
The fact that (A:) and (B:) are always ignored when assigning the drive designation is neither an error nor a coincidence, but mainly has historical reasons. Because while hard drives nowadays are becoming more and more manageable with ever-increasing capacities, you previously had to rely on completely different means to be able to store data.