I remember that the prevailing attitude of the day, especially in the business world, was "if it's not IBM, it's not a computer." So businesses bought IBM PCs when they began to look outside mainframe computing usage. And, of course, when the businessman goes home, and wants a computer there, he tends to want what he's familiar with, i.e., the same kind of machine he has at work.
Couple that with the fact that IBM 'shortsightedly' failed to patent their system, and everybody and his brother could, and did, make "PC-Clones". (I say shortsightedly, because IBM is a hardware company. They failed to see how these 'toys' would make them any real money. [sarcasm]After all, the real money is in "big iron" - mainframes.[/sarcasm] In the long run, though, they have benefited more by leaving it open.)
Since all the clones ran the same software, it grew into a 'common' system, and boosted Microsoft into prominence with it.