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.