>Hey, let's give this Pokemon an evolution.
Sure, but we need to introduce a new method of evolution. If it evolved through methods available in previous gens, it would be inconsistent for those methods to suddenly work now despite not working before.
>Hey, let's give this Pokemon a pre-evolution.
Sure, but we need a reason why the fully evolved mon was hatching from eggs in previous generations. Let's introduce incenses that make the pre-evolution hatch from the egg, as an explanation for why you couldn't get the pre-evolution in previous gens where the incense didn't exist.
>Hey, let's introduce an item that raises the defense and special defense of not fully evolved Pokemon.
Okay, but now we can't introduce any more cross-gen evolutions. If Eviolite doesn't work on a Pokemon now, it shouldn't be able to work on a Pokemon in later generations.
>Hey, let's add a new type.
Yeah, totally, dude. We can take a bunch of mons and retcon their types. How do we explain Pokemon losing weaknesses/resistances/immunities and gaining others? Fuck it. Making lore is hard.
