Throw in high-unemployment, a city which is huge physically compared to it's population (it's bigger than london with a third of the population), lots of low-income self-employed artists, writers, etc.
All of these factors combine to make it a cheap city. I'm currently staying in a 4 star hotel in a nice fairly central area for £35/night. You'd pay more than that for a smaller studio flat in london.
It’s not easy to rebound from that (after 45 years), even when you become capital, pretty much all of Germany’s successful post-war industry established itself in western West Germany. And not just industry, Post and Telekom are both still in Bonn and will probably be forever.
Berlin is also in the middle of a huge sparsely populated area. If there are no companies in Berlin that have jobs to offer not many people are going to move there.
(By the way, looking at lists with the biggest German cities is a bit misleading. Sure, Berlin handily comes out top, but look at this map: [+]. That’s not a zillion different cities like those silly natives with their tradition would want to make you believe, that’s just one city with five million people in it.)
[+] http://de.wikipedia.org/w/index.php?title=Datei:Ruhr_area-ma...
I am partly living in Berlin for 4-5 years now and now moving there with my startup completely.