Regionalism

John Kampfner in today's Times asks why the UK doesn't have a second city that matches London:

Compare the UK with other equivalent countries. Spain has two major centres: Madrid and Barcelona. Italy’s are Rome and Milan. Russia would cite Moscow and St Petersburg. Australia: Sydney and Melbourne. Germany has several, as has the United States. The only country of similar size and weight that is as over-centralised as Britain is France.

There are perfectly good historical reasons why this should be the case. Spain is the result of the union of the kingdoms of Aragon and Castile, with Madrid and Barcelona the respective capitals. Italy only became more than Metternich's geographical expression in the mid 19th Century, and unification was only completed in 1870 - Milan and Rome were two of the many principal Italian cities. Russia has moved the capital between Moscow and St Petersburg more than once, and Germany, the USA and Australia all have their origins as unions of more or less independent states. 

The UK and France are nation states (England so dominates the UK that the capitals of Scotland. Wales and Northern Ireland are inevitably over-shadowed), without the history of amalgamation that all Kampfner's examples share. It's pretty much inevitable that if a city is a nation's capital for a thousand years it will dominate culturally, politically and economically.