I'm going off of both utility and potentiel for design. Fire/Psychic not only has less resistances but also more weaknesses than Fire/Fairy. Sure yes Electric/Flying is a very good typen combo completely removing electrics costs while keeping it's benefits, Fire/Fairy again has more resistances that Electric/Flying. Not to mention design wise, Fire/Fairy and Fire/Psychic has much more variety and creativity than Electric/Flying. Also, we can take a crack at taking an unused type combo and turning it into a functional pokémon.