Not sure why, since they have no need for business class email. BBM seems to be the main driver, even though there are plenty of altenatives on other platforms.
It's definitely a London thing. They became hugely popular in the summer of 2009, rapidly making their way into the constantly shifting memepool that is the grime scene.
Witness "Maxwell D - Blackberry Hype": http://www.youtube.com/watch?v=2qtShEs5LQs
Believe it or not this isn't paid product placement. Hence he lists RIM's competitors in the intro. They probably still sent maxwell some units though.
EDIT: BBM was also heavily used as a way of sending shouts to pirate radio stations. You couldn't really ask for better marketing. It's very much waned now though.
Other stations I've heard, outside London, also give out BBM PINs - Radio City 96.7's dance music programming in Liverpool is one I've noticed.
http://www.youtube.com/watch?v=r8OhnyziTPE
If you've not seen it, it really is an example of how deep the Blackberry runs in street culture - both back home in the UK and even here in the US (take a trip through Daly City or East Bay and check out the BB's).
There was another tune called "Blackberry Swipe" about mugging people for their mobile phones. Can't remember who did it though. The grime scene really is like a musical version of 4chan when it comes to memetics.
They take it seriously too, people have been hospitalised over meme ownership: http://njaithewriter.wordpress.com/2009/04/15/itsalot-gate-g...
In Canada the Blackberry outsells Android in the teen crowd according to a report in the Globe and Mail a few weeks ago. Unfortunately, I can't find the article now.
I would argue that this is a problem, not a solution. Plenty of alternatives, all incompatible, none of which are bundled with any phone, and none of which work (or work well) on Blackberry.