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OP is right.
Charizard and Lucario both are considered "cool" mons, they have amazing megas, both design and stats-wise.
It's obvious that GF is trying to appeal to us by using them as the poster boy.
What would you think it would look like if they used, let's say Druddigon or Ferrothorn or Kecleon or Wormadam? People would react, "what the fuck is this thing?"
What would they even make out of Garbodor if they were to use it as promotion? They don't, right? It's always the edgy / cool ones (Haxorus, Excadrill, mega Blaziken) or the cute ones like Sylveon, eeveelutions, Pikachu and its clones