You answered your own question, because they want to show Dragonite as invincible and they want to showcase it more beyond just one win. This was a big capture for Iris and the writers intended to show it off in this tournament. By asking Mamoswine to win, you're asking for this tournament to be more about Dawn, which is something the writers decided not to do.Its not even about Dawn beating two female protagonists, I don't even think about that, its about Dawn getting a decent win in the tournament. We know Dragonite will lose the next match so what's the point of showing it invincible to ice attacks when its going to lose to a Rock attack in the next match?
Your logic makes no sense. Being raised from a Swinub doesn't give Mamoswine an edge over anyone. Strength is strength and we know this Dragonite has a lot of power because it was seen battling with Hydreigon and it was revealed that this Dragonite is a competitive pokemon that looks for strong opponents all the time. Just because a pokemon's experience is not shown on-screen, doesn't mean they are weaker than the ones who are. With that logic, Ash's pokemon should be stronger than everyone else's because we see his all the time. This isn't true because if it was, then Ash should've easily been able to beat Tobias's Darkrai and Latios and every other trainer for that matter. POWER IS POWER and the fact is Mamoswine is not strong as Dragonite. While Mamoswine was competing in contests, Dragonite was looking and fighting the toughest opponents and just because we didn't see it, doesn't mean it didn't happen. That's more of your own personal complaint of why the writers chose to make Dragonite strong in the first place, which is a redundant question because the writers can do whatever they please. They can make a Caterpie the strongest pokemon in the world if they want.Your bias knows no bounds. We've only seen Dragonite for 3 episodes and can't assume anything about it's experience as it was never shown onscreen. Mamoswine on the other hand was shown multiple times and was raised from a Swinub, which we all saw. Going by that matter alone, Mamoswine is reasonably the stronger of the two, but because the writers love to make Pokemon seem far more deadly then they are, Dragonite was a shoe'in to win this because of them(the writers) alone. If one were to write a reasonable story, almost everyone would likely make Mamoswine win in the end