Why in a computer there is no B drive , but there is A,C,D,E,F drives?

Traditionally A & B drives were used for Floppy drives while C onwards went to hard discs, Optical disks , removable storage etc...
If you can get hold of an original IBM PC (compatible ) with 640 K of RAM , CGA Display 4.77 Mhz 8088 CPU & Two FDD' s you will get drive A & B on the system.
The advent of HDD's made multiple Floppies redundant.

Using Subst (on the command line ) B: can be mapped to another drive or specific path if unused.