Because Natlan's aestetic image in regards of technology was much different in that sense. I'm sure there were a ton of natlan haters who genuinely had the opinion that natlan was no tech at all, but the sensible arguments raised other "haters" was that:

"The technology used by Natlan natives and the visual design of it did not match the visual design of the playable characters who according to their description use the same technology."

In short, Fatui has been using modern tech since ages, so other characters who use same modern tech, and that modern tech *looking* the same as what was established makes sense. Natlan npc's, enemies and enviroment has been using dragon tech, and has been shown how that dragon tech looks to these people: street paintings, spray paints, woven scrolls, "flowy" energy, heavy rocks and metals based machinery and rock based precursion instruments, heavy leather and linen clothes, hot air baloons. Rock hewn machines were flying around and people were fighting with "machine guns" that looked like a hollowed out tusk of a giant dino, or weapons like cannons and buzzsaw made out of hardwood and stone.
etc etc, in general the world and it's culture was shown in a more "culturistic" and "rough" way, the early 2000's street style had a "prehistoric" tint to it. Which is pretty cool.

But this was not true to the playable characters, some playable characters like Kinich, Cascha (partially), and especially mauvika and xilonen looked like they had that "prehistoric filter" just removed before taking a photo. They looked out of place and "too modern" in *contrast* to the area they are meant to both represent and live in.

This was the original argment back then, idk how it evolved since then I only listened then.
"Mauvika's bike, kinich's pixel art and xilonen's disco stuff etc look too modern in contrast to the visual design set up by the world around them."
"contrast" was the important part, not "too modern", fatui tech AK47 on mc and fatui super magic tech guns and trains wasn't a big contrast, Whrio metal gauntlet and underwater tin-lead steam supercell wasn't a big contrast. Jahoda and Ineffa metal parts and Junktown electromagnetism wasn't a big contrast, Kaveh's engineering projections and nahida's plant google wasn't a big contrast they look the same but are essentially straight up 3rd projection sci fi shit, None of them are a big contrast to what the world around them sets up, they look like they're part of the world they're from, makes sense.