Labour market and taxation are more flexible over here than on the continent. Also London was/is basically the financial capital of Europe, and the easiest destination for young talent for cultural reasons (it's easier for developers to learn English to a passable level than German, French, or less popular languages, when most of the ecosystem is built in and around English).

This said, Berlin started later than London but, from what I hear, got much hotter in the last 5/10 years, so there might be some rebalancing in progress.