The short answer, IMO is that it's (1) easier to integrate with cloud storage (e.g. NextCloud), (2) very compatible with MS office, and (3) is more mature w/r/t it's updates not breaking things, esp. w/r/t (2).
I pushed hard to get alternatives working with my NC instance, but OnlyOffice ended up being the better answer for those reasons above. Is it a full Office365 replacement? No, not if you use "advanced" features of MS office, where the definition of advanced varies greatly depending on if you're doing document work, spread sheet work, or presentation work, and where that definition definitely can include things that are easy to do in MS office but next to impossible to do in OnlyOffice. If MS office better integrated with my cloud instance (i.e. directly supported WebDav), I'd probably use it instead, but MS office really pushes integration with One Drive which I've no interest in having installed or otherwise using.
As a homelabber, I'm looking for the maximum utility solution, and at least currently for me, that's OnlyOffice. As a "bonus" OnlyOffice has a desktop variant for local file editing, should you be so inclined (although I'm trying to do almost everything in my cloud anymore).