Via Deeply Trivial, the Stack Overflow blog comes up with some answers:
Here on the Stack Overflow data team we don't have to hypothesize about where developers are and what they use: we can measure it! By analyzing our traffic, we have a bird's eye view of who visits Stack Overflow, and what technologies they're working on. Here we'll show some examples of what we can detect about each city based on one year of Stack Overflow traffic.
London has the highest percentage of developers using the Microsoft stack: while New York had more Microsoft-related traffic than San Francisco, here we see London with a still greater proportion. Since both London and New York are financial hubs, this suggests we were right that Microsoft technologies tend to be associated with financial professionals.
Just another reason why I think London and I should get more deeply acquainted.