Why do CPA firms struggle in the consulting business?

On the three-legged stool that is the CPA firm, tax and audit stand solid while consulting is perpetually wobbly. Firms seem to launch and shut down various consulting services as if by trial and error, never quite getting it right. Why? What do the McKinseys of the world have that the Big 4 don't? Is the answer to buy a successful standalone consulting firm a la PWC and Booz Allen? Or can CPA firms actually develop a successful consulting practice from within?