The thing is that Unix grew up w/o a network. Disk was slow though, so perhaps it could have had async I/O from the beginning, but... everything was slow, so maybe not. Most importantly the key is that when one is building something new, often one will build the easy stuff first, and synchronous metadata operations is definitely easier than async -- this is a lot easier to forgive 50 years ago than now because now we know that async matters a great deal, but 50 years ago it was a lot less clear.
The thing is that Unix grew up w/o a network. Disk was slow though, so perhaps it could have had async I/O from the beginning, but... everything was slow, so maybe not. Most importantly the key is that when one is building something new, often one will build the easy stuff first, and synchronous metadata operations is definitely easier than async -- this is a lot easier to forgive 50 years ago than now because now we know that async matters a great deal, but 50 years ago it was a lot less clear.