These mons were allowed in UU at one point, and were banned from UU because they were deemed too powerful. When the DLC releases in a month, many of them might be retested in UU to drop. However, because the power level with these Pokemon in it make the tier ridiculous (they were banned for a reason), there is no reason to add such a tier.Hey, suggestion there !
Couldn't you just add a tier between OU and UU? There are like 20 pokemons in the middle tier and most of them arent fit for Overused nor legal in UU, so why not just create a intermediate tier?
Many of the Pokemon in UUBL are also useable in OU in some form (Weavile, Venusaur on sun, Veil for Ninetails-A, Obstagoon, Crawdaunt, Diggersby as breakers, Primarina), even if they are not a top metagame choice.