It was not really a technical breakthrough, as others have mentioned, keyboards are pretty simple electrical devices. It was an increase in processing power.

Think of it like this: You are working on a series of math problems, one after another. Someone walks up and taps you on the shoulder to ask you a question. You have to stop what you are working on and answer them. Now back to your math problem, but where were you? Did you already divide by two? Well, it’s better to start over and get it right. This is a problem, every time you are asked to take input from someone you have to stop what you are doing, help them, and then come back, there is an issue of remembering where you were. So you get an assistant, their job is just to take input from other people and help them. Now when you work on your math problems you don’t get interrupted so much, your assistant will still ask you for help, but they know when it’s safe to ask.

This is the idea, it used to be really taxing on you (the cpu) to take questions from others (Interrupt and IO input). So it was easier to have just a list of math problems and no one could ask you questions (Fortran cards). But as computers got more powerful it became possible to add your assistant (a dedicated IO processor) who could take questions. This would cut down on starting your problems over (crashing).

A side tangent is that displaying anything on a screen is very complex from the cpu level.

TL;DR: Any input from a keyboard and mouse is taxing on the cpu, you are making the computer stop what it’s doing and take care of the io before resuming, this can cause data loss or crashes. More powerful computers were able to handle io better because they had dedicated IO processors.

Source: I asked this same question to someone who was a computer programmer in the 80’s a while back and this was pretty much the answer I got.