It's a bit before my time, but I imagine that would have broken comparability with some CP/M programs that wouldn't be aware of the concept of directories and would look in cwd. Given the era we're talking about, that seems like it'd actually be a pretty big deal. Until relatively recently, Microsoft was big on backward compatibility and it was probably a pretty significant factor in their rise to dominance.
As Raymon Chen explains the compatibility is not with CP/M but with an idiom that people still use today, namely of assuming that one can redirect to/from devices like NUL without a path prefix. There's an awful lot of existing practice that does this and doco that says that this is what to do.
Random example: Here are SuperUser answers written this year, only a few months ago, employing this idiom.
On CP/M, device names ended with ":". To me, it'd be perfectly reasonable to treat "LPT:" as a magical file present in any directory, but not "AUX" or, much worse, "aux.c", which doesn't look like a device name. Besides that, "AUX:" was never a thing in CP/M IIRC.
That doesn't convince me they actually thought this through at the time.
