I'll defer to anyone with a better understanding but I think in the case of the US it has a lot to do with attitudes about the Chinese around the time of Chinese exclusion laws. Chinese people living in places like San Francisco didn't really have a lot of options in terms of locations they could live and add to that the linguistic difficulties sojourners had would face and its no wonder they congregated. This is happening about the same time that orientalism is sweeping the country and people start visiting Chinatown to see this weird pocket of foreign culture safely lodged in the middle of an American city.

People began writing articles and guidebooks about this romanticized Chinatown experience and people began flowing in trying to see it so you have a huge upsurge(around the 1880s) in the tourist economy and it got so prevalent before the 1906 quake (in the case of SF) that you could hire a guide in the lobby of your hotel that would show you around (and the tours always seem to include watching someone smoke opium).

Guides were often initially white off-duty police officers but more and more Chinese residents started running groups through and the tours seem to be pretty formulaic since there was an amount of local cooperation since tourists meant tourist money. The issue was these tours were promoting a lot of the negative stereotypes that helped get support behind exclusion (squalor, vice, etc.) and until the Chinese merchants and social elites could start exerting more control over their image post-quake you went to chinatown to see a bunch of uncivilized exotic people that were the supposed to be the opposite of urbanizing and modernizing America. The idea of visiting Chinatown to see something kind of mystical and different persisted because orientalism is still pretty prevalent and Chinatowns encouraged it because it is a good source of money for local businesses.

Barbara Berglund and Raymond Rast both have some articles about the topic which is where I've pulled most of the this from:

Berglund's is Chinatown's Tourist Terrain: Representaion and Racialization in Nineteenth Century San Francisco; published in American Studies vol 46, no 2 from summer 2005

Rast's is The Cultural Politics of Tourism in San Francisco's Chinatown, 1882-1917; published in Pacific Historical Review, vol 76, no1 from Feburary 2007.