Windows drive lettering history explains why the main disk is C:

The convention of assigning the letter C: to the primary hard drive in Windows operating systems is a legacy of computing standards established in the 1970s and 1980s.

Historically, early microcomputer operating systems like CP/M introduced the use of letters to identify storage devices. Under this system, A: was designated for the first floppy disk drive and B: for the second. This logic was inherited by Microsoft when developing MS-DOS, which became the standard for IBM-compatible PCs.

During the era of floppy disks, many computers utilized two 5.25-inch drives: one for booting the system and another for data storage or loading programs. When hard disks were introduced in the 1980s, such as with the IBM PC XT, the system assigned the next available letter to the new storage medium. Since A: and B: were already reserved for floppy drives, the hard disk was assigned C:. This technical convention has been maintained through successive operating systems to ensure historical compatibility.