No more instant feedback. Don't want one job to affect the results of another. Memory protection and relocation added to hardware, OS must manage new hardware functionality. OS starts to become a significant software system. OS also starts to take up significant resources on its own.
Computers become much cheaper.
People costs become significant. It becomes important to make computers easier to use and to improve the productivity of the people. So, it is important to run interactively. But computers are still so expensive that you can't buy one for every person. But now, people need reasonable response time from the computer.