You're looking at a power law. Just like Cities in a country follow a power distribution of population (I just doubled checked that rule for UK and they indeed follow an almost perfect power distribution) so "Start up" areas will also follow a power distribution - but due to the extreme mobility of start-up founders and capital that will be applied globally not nationally.

Silicon Valley being the pre-eminent location for start-ups is a positive feedback loop which puts it at the extreme tip of the power distribution.