Starting from A and iterating on through Z makes sense, for an OS that's designed for two drives at most. /dev/sda and /dev/sdb are no less arbitrary than A: and B:.

One major difference was that Unix was used on big servers and couldn't fit itself onto a single disk, so /usr had to be created. DOS and Windows never needed a second drive to boot, so they didn't need to embed their resources into the drive hierarchy.

Of course, you can mount NTFS volumes at any directory you wish since at least somewhere in the early 2000s. Very few people do it, but you can!

For example:

    $Disk = Get-Disk 2
    $Partition = Get-Partition -DiskNumber $Disk.Number
    $Partition | Add-PartitionAccessPath -AccessPath "G:\Folder01"

First floppy drive was A, Second B, and when internal Hard Drives came along they defaulted to C to be compatible with computers that had at least 2 disk drives.

https://en.wikipedia.org/wiki/Drive_letter_assignment?wprov=...

[0] https://en.wikipedia.org/wiki/CMS_file_system