Because DOS used the slash as a switch specifier, aping popular DEC operating systems of its day, but did NOT adopt DEC's approach to specifying paths and subdirectories, opting for a more Unix-like approach in DOS 2.0.
How did TOPS-10 handle directories? The article says "Note that the odd numbers in square brackets [27,5434] are the TOPS-10 way of specifying directories," but doesn't give any more detail.
I am shamelessly uninterested in poring over the linked TOPS-10 manual to find out, so I'm hoping you know. :D
The numbers are a "project programmer number", or PPN. The "project number" is like a bit like a GID, and the "programmer number" identifies the user within the project (so a bit like a UID).
A lot of early operating systems didn't have hierarchical directories. It was common to have either one single directory for the whole volume (like CP/M and DOS 1.0 had), or else a two-level structure in which each user gets their own directory but no nesting of directories is allowed. TOPS-10 PPN's are like that.
Intergalactic Digital Research's CP/M was a very close copy of DEC RT-11, though not quite as close a copy as MS-DOS was of CP/M, so CP/M had a great deal to do with DEC.
VMS, which came later and is therefore irrelevant, used . to separate directory names in paths.
And Windows inherited a lot of DOS baggage.