Why computer has not B drive?

The A: and B: drives on a desktop computer are reserved in the BiOS for floppy disk drives. As most computers only come with one floppy disk drive only the A: drive will be visible in file explorer. If a second floppy disk drive is fitted then both A: and B: drives will be visible. Computers used to have a B: drive. It was the second floppy drive. Since people no longer copy from one floppy to another, the B: drive (and on some newer models the A: drive) has become obsolete.