I think it boils down to inertia.
Microsoft absolutely played their cards expertly well in the nascent days of the microcomputer, all the way through to the new millennium. They also adopted bully tactics to prevent upstarts (like Be for example) from upsetting the apple cart and had ironclad contracts with OEMs when distributing Microsoft Windows.
It also helps that the IBM PC became such a pervasive standard. The competition, such as Commodore, Atari, Acorn, you name it, couldn’t help but find ways to blow their own feet off any time they had an opportunity to make an impact. Heck… even Apple came very close to self destruction in the late 90s, with Steve Jobs and NeXT being their Hail Mary pass.
In short, now that we’ve had personal computers for decades, it’s very difficult to break into this market with a unique offering due to this inertia. You have to go outside normal form factors and such to find any interesting players anymore, such as the Raspberry Pi and GNU/Linux.
As an aside, I kinda wish Atari Corporation was run by less unscrupulous folks like the Tramiels. Had they realized their vision for the Atari platform better and hadn’t taken their “we rule with an iron fist and treat our partners like garbage” mentality with them from Commodore, they might have stuck around as a viable third player.