This seems like an okay overview: https://brattahlid.wordpress.com/2012/05/08/is-docx-really-an-open-standard/
Basically ODF was already a standard and perfectly sufficient to tweak for anything Word would ever need but Microsoft knew that they keep their monopoly secure, in part, by making other software fight to be compatible with Word documents. They also prefer to be in control of the entire ecosystem rather than implementing a shared standard. They proposed docx and other -x formats as their own open standard and were rejected until they more or less bought off ISO through donations and committee positions.
Microsoft then proceeded to make their own proprietary tweaks anyways, making it still very difficult to support the docx format.
This seems like an okay overview: https://brattahlid.wordpress.com/2012/05/08/is-docx-really-an-open-standard/
Basically ODF was already a standard and perfectly sufficient to tweak for anything Word would ever need but Microsoft knew that they keep their monopoly secure, in part, by making other software fight to be compatible with Word documents. They also prefer to be in control of the entire ecosystem rather than implementing a shared standard. They proposed docx and other -x formats as their own open standard and were rejected until they more or less bought off ISO through donations and committee positions.
Microsoft then proceeded to make their own proprietary tweaks anyways, making it still very difficult to support the docx format.