Me: *hastily attempts to shove the backstories of every single Lodger into a two page B plot scene* *fails*
The majority of the Lodgers are either immigrants or recent transplants to London. Jekyll met a couple of those immigrants during his travels (such as the trip Lanyon mentions at the start of this chapter). But most of them came to London at some earlier date, seeking career opportunities, better living conditions, or--in a couple cases--fleeing active persecution of their particular brand of rogue science. London has always been a major port city, drawing people from all over the world, and in the 1880s it was one of the most important economic capitals of the world. There's no better place to gather a band of nerdy misfits together.