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Megas were functionally an expansion to the roster of Pokemon, which is what people really care about. The new designs, new models, new stats, new Abilities, etc. It spawned all kinds of fanwork and speculation about their favorites, talking about which Pokemon should get one next. And some of them, like Beedrill, Mawile, Pinsir, etc. drew in new attention and fans for 'mons that nobody cared about raising previously.