* Internet Explorer came bundled with the OS and therefore had a leg up on the competition. They made it impossible to remove that browser, so people targeting Windows would likely also automatically target IE in the knowledge that it would be available on the system.

* Microsoft controlled the Windows API and appeared to be sabotaging attempts to build cross-platform applications. And they seemed to think it was substantial interference. Take this for example:

"Microsoft's videotape showed the process as being quick and easy, resulting in the Netscape icon appearing on the user's desktop. The government produced its own videotape of the same process, revealing that Microsoft's videotape had conveniently removed a long and complex part of the procedure and that the Netscape icon was not placed on the desktop, requiring a user to search for it. Brad Chase, a Microsoft vice president, verified the government's tape and conceded that Microsoft's own tape was falsified." [0]

If MS had restricted itself to a banner on their website saying "Download and Install IE!" they'd have been fine; the issue was that they were creating an environment to destroy Netscape for no good technical reason, they just wanted to destroy Netscape. And that was part of a pattern of behaviour where they were employing consistent tactics against anyone trying to create cross-platform software.

[0] https://en.wikipedia.org/wiki/United_States_v._Microsoft_Cor....