1) Stanford (a top research university, with world-class students and professors)
2) Money (investment money, from a lot of people who got rich from tech)
3) A high concentration of geeks/techies/developers/whatever you want to call them
No place I know of in Europe has all three of these (in fact, I don't think any place in Europe has 3) alone).
EDIT: IMHO, the closest thing Europe has to these is actually Zurich: it has top universities, money and big companies all in a small geographical area.
By github usage, which seems a decent proxy [1], London is second after San Francisco, followed by Paris, and Berlin is only 7 (much smaller city though, so density could be higher).
[1] http://www.itworld.com/cloud-computing/376551/where-world-ar...
National capitals have defence contracts and government IT, which so lucrative it sucks a lot of talent away from industry. London is also a financial centre, so that's even more competition.