Disclaimer: I work for Microsoft (Azure) but have no relation to Office or Microsoft Teams.
It is about the technology platform, because the amount of resources necessary to ensure feature parity is a function of the platfrom. Word for Windows and Word for Mac are probably two separate giant monsters of early-90's C++ and keeping them in sync is agony. Assuming Teams is written in Electron, the amount of work is nowhere near the same; not having feature parity across operating systems is probably harder than having it.
Part 1: https://www.youtube.com/watch?v=3HROqnw-nf4 Part 2:https://www.youtube.com/watch?v=MGMoRu5yrVc
Still, no matter how nice that codebase is, I assume they can't beat Electron for ease of X-platform compatibility.
Our small team within a giant org is using hipchat, but it's near impossible to get anyone else from the org on there for shared projects. Maybe they'll be more receptive to MS Team.