Why do both the Conservatives and Labour push so hard for cuts to taxes and public spending when the British Social Attitudes Survey shows only 1 in 10 Brits want this?

A graph from the 2013 British Social Attitudes (BSA) Survey shows fewer than 1 in 10 Britons have wanted this in the 30 years since 1983. More than 90% have consistently wanted the level of tax & spend to remain the same or to increase.