There was an article in the Evening Standard on the day the 507 and 521 were replaced by smaller buses on the chaos that occurred at Waterloo despite the newspaper being a huge Boris supporter.
London TravelWatch campaigned to save them.
http://www.standard.co.uk/news/now-travel-group-wants-to-save-the-bendy-bus-6811912.html
Most places in Britain don't use them because they don't have the numbers to support using them - the same reason many places see few or no double deckers.
I've caught them on multiple local routes in Bradford, Leeds and Sheffield without going any great distance from my home. They were used in Sheffield as long ago as the 80s.
They currently run every 7-8 minutes between Bradford and Leeds on route 72.
London doesn't have very few. It has none. Why is London so different from Bradford, Leeds and Sheffield that it has not a single route suitable for a bendybus?
Unfortunately as London has ruled them out for now, many manufacturers are not designing articulated buses for driving on the left as the remaining markets are quite small.