You have to think in historical context. Many of the Chinatowns in North America were where the Chinese immigrants (or the native-born for that matter of fact) were pretty much allowed by those in control (pretty much the "whites") to settle during the anti-Asian hysteria of the 1800s and even until the early-mid 1900s. These were often the oldest and/or oldest parts of town. The building stock as a result is much older.

I was in Victoria, B.C. recently and noticed that since heritage buildings (i.e., anything not built since the period inbetween WW-I and WW-II) are in vogue, the Chinatown there has become trendy as has the one in Vancouver (though there is a lot of redevelopment in the latter). Back in the '60s drive toward suburbanisation, there were proposals to flatten Vancouver's Chinatown to run freeways through it. Activists stopped it (which is why there are no freeways into Vancouver) though a predominantly African-Canadian section of town was flattened to build the only parts of the freeway (the Georgia and Dunsmuir viaducts).

As for London's Chinatown, those who were around in the '70s will remember it as being a little seedy (full of sex shops/shows, massage parlours and prostitution). I wouldn't call it a slum like east London and even Notting Hill/Ladbroke Grove were more like at that time. The buildings may have been cleaned up now but they are still, in general, the buildings from just after WW-II if not before. Just cleaner now that Soho is a touristy entertainment area.