You can have something like :
- The famous Art Deco region of Junction City, New Planetia
- The Hung Gardens of New New Las Vegas (OK, that's weird)
- The Adobe Walls of Adobe City (bound to happen)
But those will be local, not on the scale you want.
I want there to be a few regions, which have homogeneous styles. As one example, I'd like to have an Art Deco/Dark Deco region. All of the buildings would fit this theme, most fashion would fit this theme, and most consumer goods would fit this theme.
I know you want reasons why this could happen, but it could not realistically happen on such a large scale.
First and foremost people are individuals and if experience tells us anything it's that telling them they have to conform will immediately generate resistance. This is particularly the case with anything associated with personal identity.
The primary reasons why people build and create different looks in Earth cities (and that's just one planet !) are :
- Climate - different architecture for different conditions
- Historical development - some styles relate to local culture and keep being "echoed" in modern designs because many (but not all !) people like tradition
- Money - some people like to flash the money around, some don't and most cannot !
- Cheaper availability of different materials in different places
- Habit and expectation - As strange as it sounds we have different habits and expectations from our local architecture and homes and offices. For example, in North America screen doors are all over the place, but in my home town (Dublin, Ireland) I've never even seen one ! Mostly that's a habit and an expectation, not a practical issue.
But these differences happen on a very small scale, not just on a larger scale. Different roads can be in different styles, never mind planets.
There are some reasons why local areas may enforce a look or appearance :
- Historical preservation - an area may be a tourist haven and people don't like to rock that financial boat or there may be legislation preventing you from changing the appearance of locale (visit the UK and see this in operation).
- Too hard or expensive to change - typically tower blocks are like this and await major economic developments to change them.
But these (again) are local issues, not global.
For buildings & architecture, the best I can come up with is that buildings constructed in such a matter would be given a generous property tax credit. Non-compliant buildings would not be given the credit, and they'd be hit with a heavy tax penalty.
This doesn't make sense unless your government gets something out it. Why would a government care about the appearance of anything (I know mine doesn't) ? So I don't think this works.
It also runs into a problem if people are in a region where a particular style is just not practical. Live in a hot region and then try living in a suitable home for that region in a cold region - bad plan.
For fashion, I imagine it would mostly be upheld by cultural standards, and that domestic clothes designers would keep the trends going. I also imagine that the natives would consider radically different styles to be low class, scandalous, or degenerate.
Might work for fashion, but will not remain constant over a large time or space. When I was growing up my mother (by no means a conservative) would not have left the house without a scarf on her head, but that changed when I was still young. It will doubtless change again.
People just don't stay that constant and rigid and some people will always push on any limit and no matter what eventually that limit will move a little.
For imported goods, a tariff could be applied to items that are "out of style".
Tariffs beget tariffs.
Meaning that if you impose a tariff on someone bringing goods into your economy, they will immediately impose a tariff on you bringing goods into there own.
And tariffs will be changed in response to the whim of the people, even if that people was a single ruthless dictator. So this cannot remain constant either.
Another answer said ...
The IKEA mega-corp sells standardized flack pack easily deploy-able buildings and infrastructure.
What would happen ? Well in no time some enterprising local entrepreneur would decide they could make a lot of money selling something else. And they would ! Locally sourced materials and labor - everyone likes that !
So flat pack might do for some people, but not everyone and that sort of thing becomes a status symbol. So people start customizing their property simply to look different and gain some kind of status boost.
Anything else, costs more and longer to produce and is usually of a lower quality.
And that would actually make it more likely people wanted it !
But there is a more fundamental reason why this won't work : even IKEA sell a variety of appearances. They do this because people want different things and are willing to pay for it. They do it because every new season brings people wanting "the latest" look - even if it's just paint on the wall or wallpaper or floor coverings or this year's fabulous new garden gnome design. People like new things, because ... well, they're all mad, I think. :-)
So this won't happen (realistically) on the scale you want, not even on a planetary scale and probably not even on a city-wide scale in the long run.