Correlation without causation. The United States has a lot of "the largest" by an order of magnitude. The size of the intelligence community could be causal, but so could the country's national visibility or wealth consolidation.

We're also working with a messy definition of "terrorism." For all its problems, the US doesn't have a destabilized central government; I would imagine one could ask the leadership of countries undergoing active revolutions whether they consider the opposition "terrorists," and if they do, I'd say they have a bigger "terrorism" problem than the US.