I think you head the nail on the head when you said there were no IBM-exclusive chips. I think the openness of the system followed by the proliferation of clones drew a ton of companies and developers to the platform.

The IBM also looked like a business machine whereas others felt cheap. It may not have looked like much on first glance but they struck a good midpoint between the C64/Apple etc and the business-oriented minicomputers on the higher end of the scale.

I think the success of the 5150 was less about what it did and more about what it was and what that represented, particularly to businesses

BYTE had the best magazine covers