Why are there Pokémon with Hidden Abilities that aren't obtainable?

Question

Chandelure's Hidden Ability, for example, is Shadow Tag, but it's not possible to get a Chandelure with that ability. ???

Answer 1

Game Freak likely intend to release those Hidden Abilities; they just haven't done it yet. They probably want to release a few abilities every so often so there is always something new to bring into play.

By the way, this Generation changed Chandelure's Hidden Ability. It's now Infiltrator, not Shadow Tag. So Shadow Tag Chandelure will never be released. ;-;