London is a very wealthy city often acting as European HQ for many companies based outside Europe, with huge amounts of business / commerce and generates vast amounts of premium class traffic for people whose time is valued extremely highly.
Berlin is a tourist / Govt city, with lower incomes and tending much more towards economy class tickets.
London City airport survives because of the tidal wave of money sloshing around London. Germany is wealthy but as the mayor said, Berlin is poor.