Why hasn't Yelp worked outside the USA?

This chart shows the total reviews created for the top 25 venues in each category in London and in New York. It’s a pretty clear picture: people in NYC use Yelp and people in London, by and large, do not. I’ve combined Yelp and the European rival Qype and even so the gap is huge. There’s a similar picture if you look at other European cities. And if you look on the forums for Yelp UK, a substantial part of the traffic is clearly from American expats. 

Transient

I should point out that this isn’t a coverage gap - there are lots of places reviewed - but an engagement gap - places just have far fewer reviews. 

Yelp launched in the UK back in 2009 and smartphone penetration in the UK is not substantially below the USA. So what is it? ‘Cultural’ is easy, but a cop out. Certainly, people in US cities do eat out more, but is that enough? Has Yelp just not pushed hard enough? Or maybe Americans are just keener to share their opinions (cf Zagat).