>>44404968In real life, it's because it's a shillmon.
In-universe, it's because Mewtwo is made of both Mew and human DNA, and each evolution pushes it further in one direction than the other.
In Charizard's case, it's a domesticated dragon that lost its draconic power in the process. Zard X brings back its dragon power, while Zard Y cranks up its domesticated traits to 11.