Because they dont want to...
Its not free. Microsoft has to PAY for each title included and if microsoft wants to cut costs they can drop regions were the cost is to high for a title..
Frontier is not a charity if microsoft can't agree the correct fees, games get dumped in regions where low returns are expected.
The game never release on Xbox store in turkey and many other regions... So microsoft has no licence agreement for the title..
Its not just microsoft all companies are profit first even playstation... but just like they don't bother with language conversion for the region which cuts costs...