Likely because of regulations. Rewards in Germany must be declared for tax purposes, thus it would require an overhead work for the custodian and for Brave.
Wow, if even Germany is out from the supported regions, I think mine will follow soon the same path, hope not but anyway, right now brave has to finalize the other way to withdraw BAT that is talking about these last months, not just for european and asian users, but for the project itself.