How about because the decision to ban them was based on the premise that the current UU metagame is both stable and truly reflective of what UU stands for ... when in reality it is neither.
There were not banned on the premise that UU is stable and i'm not sure what your getting at with the mention of what "UU stands for" since UU is simply what it sounds like "underused". Pokemon that are not OverUsed are UnderUsed and those deemed too powerful for OU are BorderLine it's as simple as that.
Houndoom and Empoleon were banned on the premise that people thought they are too powerful for UU and thus would create even more instability in the tier, but by no means did anyone say that the tier was balanced or actually stable.










