Because Microsoft leveraged a monopoly (Windows) to gain another monopoly (browser), which is illegal. App stores didn't exist prior to Apple's. Also, Apple's store didn't work on competing phones (not that there were any at the time), nor did it work on either macOS or Windows, it only worked on Apple's iPhones. It's like Amazon controlling what Kindle books exist, or Disney controlling what you can see at Disneyworld or buy at Disney+.