What was a key element in making computers widely accessible to the public?

Not much.

The big breakthrough in making computing accessible to all was the Personal Computer, which was developed into the mass market by IBM. There were PCs before the IBM PC, but they were not successfully marketed at a huge scale.

IBM bought the disk operating system, the software to make the machine work, from Microsoft. Microsoft had itself bought the system from Seattle Software Works and they just licensed it to IBM.

There were other disk operating systems out there, including the very popular CP/M and MS-DOS was more or less derived from and very similar to CP/M.

The IBM PC took off spectacularly, and Microsoft became very rich. But their flair was (and still is) in marketing, not in invention. Mice, Windows interfaces, desktop images, icons, etc. etc, were invented elsewhere, some by Xerox at their Palo Alto research centre, some by Apple, and quite a lot from the various Unix flavours out there.