Because legacy reasons.
In the early days of personal computing your computer wouldn't have a hard drive. All it had was a floppy drive, drive A. If you where fancy, you had two good drives, drive A and B.
When hard drives became more common, it would mostly be drive C, the third drive in the system.
And to preserve that functionality and to be compatible with programs that assume drive A and B to be floppy drives, those letters are not the default for the first harddrive.
You can however force windows to mount any drive to the letter A or B or any drive letter up to Z.