Going off of what Peter said, it also is because Hong Kong was, "owned," (I guess it is the correct term lol) by the British at the time and it has been noted in many places (possibly even in the documentary above)-not to turn this political- that in HK where the British had no interests, they left it to it's own devices; Kowloon is an example.
It could just be that these fictional Qrs. represented this state of indifference, hence why they're falling apart and dirty (but the living spaces off all these people are super clean and well-kept). Whereas, like Peter noted above, where there would be businesses and commercial areas (places where the British would undoubtedly have interests), there is cleanliness and upkeep.
Go to Venice or Rome and you'll see the same thing, for the most part; don't forget, these cities and locations are among the oldest in the world and many things that were built hundreds of years ago, are still standing without the upkeep (which is a testament as to how well things were constructed back then).
With upkeep, they'd look just like your own town or city, albeit with a much more classical and less-modern look.