Because Accuracy and Evasion are set stats, that can only be raised and lowered above the average by moves intending to do so.
But mainly, Gamefreak would look at that as them being obviously over powered, but Ancient Power only increases your stats that you use consistantly, you don't have an Accuracy stat, Moves like Hone Claws and Coil increase the accuracy for your moves.
You can also notice, that no moves Raise Accuracy or Evasion as a secondary effect, it's a bit of a Gamefreak thing.
But in all, Accuracy and Evasion, Aren't Stats They're already set and same for every pokemon, and are changed through moves like that, while also, It would make Ancient Power rather Overpowered