Here, I'll provide an example: Galidor.
Number one reason that Galidor failed, set-wise: sets were incompatible with anything else (up until 2002, when the ratchet joints became commonplace). The same thing applies to these minifigures. Take for example the old door piece. A woody-fig is just as tall as the door. Add on a Mask, and they won't fit. Another example, the chair piece: the woody-fig can sit on it, but their legs stick out. Those are the only examples, piece-wise, that I can think of offhand, but I'm sure their are more.
Lego is a single system that combines a world of different themes, allowing kids to have their Harry Potter characters go up against a Jedi, or a Ninja from Ninjago going to another realm to say hi to the Chima guys. You can't make the main characters of your theme, the ones that will appear the most in sets, incompatible with everything else Lego does.