I guess because they require development that the developer hasn't done? There are so few W8 phones out there that most developers wont consider it. W8 itself is still worth a go as it will have tens of millions of users already.
If MS is the developer or funder, its probably the same reason there are never any new xbox windows phone games, their focus is on the new Xbox and that project will be swallowing millions by the day and would be disastrous for them if it failed.