The Austrian military didn’t just adopt LibreOffice; they actually contributed back to it. Over five person-years of development work went into adding features they needed. Those improvements are now available to everyone using LibreOffice, which is pretty cool.
Might be the licensing type.
Might also just be what was/is known at the time to the decision makers. I’m in the IT world and didn’t know/learn about ONLYOFFICE until this past year.