Of course, the reason they got better so fast is volume. There was just way more investment into those platforms. Which means this explanation is somewhat circular: they were successful because they were successful.
I think a more useful explanation is that people rate the value of avoiding vendor lockin extraordinarily high, to the extent that people will happily pick worse technology if there's at least two competing vendors to choose from. The IBM PCs were not good, but for convoluted legal reasons related to screwups by IBM their tech became a competitive ecosystem. Bad for IBM, good for everyone else. Their competitors did not make that "mistake" and so became less preferred.
Microsoft won for a while despite being single vendor because the alternative was UNIX, which was at least sorta multi-vendor at the OS level, except that portability between UNIXen was ropey at best in the 90s and of course you traded software lockin for hardware lockin; not really an improvement. Combined with the much more expensive hardware, lack of gaming and terrible UI toolkits (of which Microsoft was the undisputed master in the 90s) and then later Linux, and that was goodbye to them.
Of course after a decade of the Windows monopoly everyone was looking for a way out and settled on abusing an interactive document format, as it was the nearest thing lying around that was a non-Microsoft specific way to display UI. And browsers were also a competitive ecosystem so a double win. HTML based UIs totally sucked for the end users, but .... multi-vendor is worth more than nice UI, so, it wins.
See also how Android wiped out every other mobile OS except iOS (nobody cares much about lockin for mobile apps, the value of them is just not high enough).
I think a more useful explanation is that people rate the value of avoiding vendor lockin extraordinarily high, to the extent that people will happily pick worse technology if there's at least two competing vendors to choose from. The IBM PCs were not good, but for convoluted legal reasons related to screwups by IBM their tech became a competitive ecosystem. Bad for IBM, good for everyone else. Their competitors did not make that "mistake" and so became less preferred.
Microsoft won for a while despite being single vendor because the alternative was UNIX, which was at least sorta multi-vendor at the OS level, except that portability between UNIXen was ropey at best in the 90s and of course you traded software lockin for hardware lockin; not really an improvement. Combined with the much more expensive hardware, lack of gaming and terrible UI toolkits (of which Microsoft was the undisputed master in the 90s) and then later Linux, and that was goodbye to them.
Of course after a decade of the Windows monopoly everyone was looking for a way out and settled on abusing an interactive document format, as it was the nearest thing lying around that was a non-Microsoft specific way to display UI. And browsers were also a competitive ecosystem so a double win. HTML based UIs totally sucked for the end users, but .... multi-vendor is worth more than nice UI, so, it wins.
See also how Android wiped out every other mobile OS except iOS (nobody cares much about lockin for mobile apps, the value of them is just not high enough).