If you scroll down, you will see Gen I priority table, they had +6, +1 and -5. That's it.
In Gen II, they had new moves, which took new slots. This keeps going until Gen V, where there are moves for every priority bracket except -2
There really is only one way to look at it: They have not made the move yet. Just like Gen I to Gen V, new moves took new brackets. Maybe Gen VI will give us a -2 priority move.
We can only Dream...