Policy - policy rubber-stamped by Livingstone and continued by Johnson.
This new generation of towers isn't necessary if it's deemed okay that London begins to lose its pre-eminence as a 'world city'. The argument is these towers have to go somewhere - London, Paris, Frankfurt, Berlin - because they're European or World HQ's for multi-nationals and this is how multi-nationals want to work.
If that is the case, I suppose you have to think about how many people work in these towers and how much corporation tax those companies will pay, and that other companies might be tempted to follow to the city where this tower goes. It will be interesting to see who ultimately occupies the space to see if that argument holds water. Of course some might think the price not worth paying, even though London is now so vital to the national economy.
Personally, I think there might also be a strategic element of giving 'the city' a major kick start south of the river - currently it's almost as if the Thames is an impossible barrier to surmount.