I don't think Microsoft was acting "not nice" - most of their extensions were submitted to the W3C (and even ActiveX was standardized by X/Open, however pointlessly). However, the major vendor, Netscape, simply was not going to adopt any IEism under any circumstances.
The biggest issue with being frozen on IE6 was not so much the standards but all the terrible bugs.
Keep in mind that at the time, Netscape was even more proprietary than Microsoft. (Layers instead of DOM, JavaScript SS instead of CSS)
And if you want to get technical about it, in theory you could support ActiveX on any platform. The problem was the controls were all Win32 software. There probably would have been some marginal benefit for Netscape to support it on Windows.
The biggest issue with being frozen on IE6 was not so much the standards but all the terrible bugs.