On the programming side, there wasn't either the
memory capacity or
processing power to implement a modern disk file system. One of the
first computers I worked with was a System/360 model 25 running
DOS/360. The machine had 48K of core memory, 12K of which was for the
OS, leaving 36K for programs. No virtual memory.
Unix was a counterexample. Recall that v1 worked on a 24K
machine, 16K of which was OS and 8K user. And it had a modern
file system. Programming was so much easier that it lured
people (e.g. me) away from big mainframe computers.
Doug