[BillyTheMage]

I'd guess it's simply because most people have never heard of it.

I just downloaded it to try it out, and it looks really nice, although it completely ignores my system theme stuff so it looks extremely ugly, but that's beside the point because LibreOffice is far uglier.

There's also the fact that OnlyOffice is made by a company for profit, and that turns a lot of FOSS away. Many, including me, are into FOSS because of some underlying anti-capitalist reasons and avoid it out of principle (though I'll admit I simply didn't know of its existence).