Because paths don't have to start with a drive letter – a path like \foo\bar is relative to the current drive.
"DIR \W" does a DIR of the "\W" directory on the current drive.
"DIR /W" does a DIR listing in wide format.
"DIR \W" does a DIR of the "\W" directory on the current drive.
"DIR /W" does a DIR listing in wide format.
"DIR/W" is the same as "DIR /W".
Which would have made it impossible to determine whether you want to invoke the command "DIR" in the current directory, or the command "W" in a subdirectory named "DIR".
Of course, they didn't use unixy paths for directories. A fully-qualified file name would be something like (and it's been decades so if I get it wrong forgive me) DRA0:[SYS.USERS.BREGMA.PROJECT.SOURCES]HELLO.C;1 and anyone who was sane would use logical names in DCL to make things readable.
(Another key difference is that on UNIX the shell expands '*' before passing it to a program, but CMD doesn't so each program has to do its own globbing)