I feel the same here in Brazil. Facebook (and Whatsapp) groups are deeply ingrained into culture, so to make a community outside of those standard tools, you have to work 3 times harder.
For small groups, depending on their ideology and objectives, it may not make sense to pursue such a hard change.
However, for companies, the trade-off is very different, and some extras from Discourse can change the balance.
The main community for a bank here in Brazil was made by non-employee that was enthusiastic about the brand. The actual company started participating and the community was almost official and big. A competitor approached, bought the Facebook account of the owner and made the community be only about moving to their product 
. This is the same thing that happened to Dota forums
In Discourse you own the data, can move it anywhere, query it, restrict access to customers for some categories, monetize with ads, and all that everyone here already knows. Most companies know that they can’t expose their customers to the wills of a different company that owns their nothing.
In the end it depends if your company considers this Japan community important enough to invest time and efforts into building it into a proper platform, or if it doesn’t really matter and can be outsourced to a third party.