I noticed that Tyranitar’s the only pseudo-legendary to not be used by a final boss in its introductory games; any guesses as to why they didn’t give Lance one in GSC/HGSS? It’s reasonably draconic-looking (hey, they made Tyrantrum a Dragon!) and it’s quite powerful and God knows Lance could do without one of his blasted cheating Dragonite and get some variety in his team! Similarly, any thoughts as to why they didn’t give Steven a Magneton, a strong Steel Pokémon that was available in RSE/ORAS?
You know, I’ve never thought about it… I think there are several plausible reasons, though. First, in most generations, there’s someone new who fills that role, whereas Lance was an established character already and Dragonite was firmly set as his signature Pokémon before Gold and Silver happened. And this was only the second generation, so it’s not like this was already a rule or convention by that point; in Red and Blue the final final boss was the rival character, not the guy with the Dragonite. And maybe the very fact that they didn’t give Lance a Tyranitar, in contrast to the way they’ve behaved more recently, says something about what they think Tyranitar is supposed to be – that is, maybe there’s something about him that strikes Game Freak as particularly not draconic, possibly because he’s basically a dinosaur and therefore needs to be a Rock-type (yes, you can compare Tyrantrum, but that’s fifteen years later, and I don’t know if I’d expect 100% consistency over such a long period on something as vague as what the Dragon type is; they may have different ideas about it now). I also kinda think maybe they had in mind for Larvitar and its evolutions to be almost like a “secret” Pokémon – Larvitar doesn’t show up until the very end of the game, at Mount Silver, and neither it nor its evolutions are ever used by any trainer prior to that. Misdreavus is the same. I think that’s part of a deliberate choice, meant to emphasise the remoteness of Mount Silver – there are Pokémon there that you won’t even have heard of during a playthrough of Gold or Silver, and Larvitar, Pupitar and Tyranitar are part of creating that aesthetic.
Oh, right, and you asked about Steven and Magneton. Well, I think they very easily could have, but it would have been difficult to put together a full team of only Steel-types, since Forretress, Steelix and Scizor weren’t available in the original Ruby and Sapphire. The only other Steel Pokémon around is Mawile, who is rubbish before generation VI. The Pokémon that they gave Steven instead – Cradily, Armaldo and Claydol – fit in with his geological pursuits, so they’re good thematic choices, even if they don’t actually match his specialty type. Going with three Steel-types and three geological/archaeological Pokémon, splitting his team down the middle, gives him a bit more flexibility in responding to threats, and makes it feel less like the non-Steel-types are somehow a mistake; they’re clearly meant to be there.