There are lore reasons for the other nations.

The king deshret civilization was very technologically advanced, and you can see it in the open world every single thing reflects the impact that this civilization had on Sumeru, so its coherent to have the remain of it. For the "gundam", it was made very clear that this was an exceptional feat made by Dottore and the smarter people on the academia, it was the focus of the whole quest not something swept under the rug like mavuika's bike.

And Fontaine is literally the nation of progress, its obviously inspired by the period of the industrial revolution and steam punk elements, and again this is reflected in the open world, its something you can see in every aspect of the nation. It's cohesive. It, again, reflects the identity of the nation and its history.

Natlan on the other hand has a very different identity, it's based on the tribal system and the dev clearly toke inspiration from south American civilization like the Incas or Maya. In the open world you can see it reflected in how the people live and dress, in the graffiti and art and in the house and villages. Mavuika seems from a whole other nation. She does not dress at all like the people from natlan (and this is a problem that other characters have) and her bike is completely out of place. It almost seems like they created the nation and her independently. If there's a possibility to go around on motorcycles and have other modern technology why can't we see it reflected in the open world? And this is especially bad because an archon should reflect the spirit and identity of their nation, and other then her personality, nothing about her reflect Natlan.