It's also a fact that IBM was not interested in a private PC market at the time. They laughed at Bill Gates when he went to them, asking them for support and suggested that it would revolutionize the PC World. They were only interested in Corporate and business/office applications.
As far as the market share, MS still outnumbers Apple by over 10 to 1. (Setting the iPhone thing aside.)
Check out a recent movie called “The Pirates of Silicone Valley”. It explains the history nicely and was well done. Anthony Michael Hall played Bill Gates in the movie and did a great job of displaying his nature.
That's what all of the companies working under that business model did. Apple made Apple, Atari made Atari, Commodore made Commodore. But Apple did branch into making Windows programs later.
Most software program manufacturers had to make theirs able to work with Apple or IBM, not both.
Back in the early 80s, you'd choose to write your software for IBM (DOS or CP/M), Apple, Atari, Commodore, TRS-80, etc. Some companies, like Microsoft, wrote software for most of them.
They laughed at Bill Gates when he went to them, asking them for support and suggested that it would revolutionize the PC World.
Not quite how it happened. IBM went to Kildall of CP/M fame, then went to Microsoft, for an OS for its new line of machines. At the time, IBM didn't consider software to be very important, or worth much. Gates licensed QDOS from another company, then worked with IBM to customize it for the new PC. Somewhere in there Gates bought QDOS outright without telling the owner that he had IBM wanting it on the other end. Only paid about $50,000.
Thus the PC was released with MS-DOS as the standard option, and it became the de facto OS for the PC. Consequently, after Compaq reverse-engineered the BIOS, the flood of clones all needed to run MS-DOS to be compatible with most of the PC software out there, and Gates made his fortune.