I believe, the main problem is that office suite use is kind of niche. While I was still going to school, I needed one all the time, but now I practically just don't.
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It's all well and good to know about alternatives, but what does onlyoffice do that would make it a compelling choice over libre office?
It has a pretty nice UI, a solid darkmode (libreoffice's can clash sometimes), has better Docx compatibility than libre generally, and has a tab view for multiple open documents.
I like it better than libre, personally, but both are good enough for most people.
…To start with, the major problem with ONLYOFFICE not being known to more users is its way of marketing. When you head to ONLYOFFICE's website, you find it pitched as a collaborative office suite tailored for enterprises or small businesses. So, most users consider it to be fit for enterprises and end up not trying it ever.
Moreover, if you head to the download page on their website clicking on the primary "GET IT NOW" button on the homepage, you find the enterprise version promoted first.
It does not even say that it has a desktop edition, unless you either head to the footer of the website or navigate the main menu again to look for the icons…