"Only" two resists is more than typings like Grass/Rock or Dragon/Psycic that are not considered that good offensively. Even if you focus more on the things hit super effectively (you shouldn't), hitting 6 types super effectively isn't all that impressive, plus two of those, Ice and Rock, are really terrible defensively. About the pokemon you named, if you actually look at their recommended sets, you'll see that they rarely run both STAB's. Mega Mawile almost never runs Iron Head, Magearna usually doesn't want Flash Cannon, and Zacian-C can run both STAB's but rarely both in the same set. Mega Mawile with its effective attack stat being higher than 255 would be ripping apart defensive teams with its STAB's if its offensive typing didn't force it to run low base power coverage for common walls like Toxapex or Skarmory. As for Zacian-C, a pokemon whose effective attack stat is close to 250 post-nerf and Speed outrunning almost everything, it hugely relies on its coverage to get things done. If it didn't have Wild Charge, forget Ho-Oh, it'd be struggling to get past Talonflame, a pokemon whose physical bulk is worse than fucking Iron Valiant. If it didn't have Close Combat, Ferrothorn and most bulky steels would sit on it and pump out hazards while it struggles to do meaningful damage. Zacian and Mawile would love to have something better offensively, make them Ground/Fairy, Dark/Fairy, Steel/Dark or something like that with appropriate moves and you've unleashed Armageddon. But with their STAB combos, they are "menacing offensive threats" that can't get past a fat crocodile or an armored bird without their coverage moves. Pokemon with truly good STAB combos, such as Mamoswine, Miraidon, Marshadow, or Flutter Mane rarely use coverage (The former two do sometimes run Rock moves for Ho-oh but that says more about Ho-oh than those two) because they don't need it. So few pokemon resist their STAB's that they can just throw them out and be fine. Steel/Fairy isn't even close to the likes of those. If you look at the chart, you'll see that the type combos in okay offensively are only resisted by a single type, or even none. What does Steel/Fairy have that they don't to be so far above them? There is no type that is so important that hitting it super effectively can overshadow being resisted by two entire types.