IE used ActiveX. Lots of websites back then used ActiveX because IE was "the standard". ActiveX was MS proprietary, so other browsers couldn't use it. This made many websites only usable on Windows.
Wrong. ActiveX ran binary code directly on the machine. You can't do that if you're not on a Windows platform. I suppose maybe you could have used ActiveX to run Sun binaries on a Sun platform, but how useful is that? Any website is only going to have Windows/x86 code, and will fail on a Sun or other platform.
And no, at the time, many other browsers were not available on Windows, as they had a tiny market share, largely due to how incompatible they were with sites.
ActiveX failed eventually because of the rise of Javascript, and also because ActiveX was a giant security nightmare.
Do I need to spell this out any more for you?