Coronary angiogram vs. CTA - why is one chosen over the other?

A friend of mine (53M / 5'6" 160lbs.) recently did a treadmill stress test due to a problem with his heart racing from time to time and intermittent (mild) shortness of breath upon exertion. Prior to this, he had worn a Holter monitor for a week. His cardiologist said there was nothing noteworthy from the readout. He was able to complete all three stages of the treadmill test with no problem.

When he saw his cardiologist about the results, he was told that he might have a small blockage at the bottom of his hear but he wasn't 100% sure from the the stress test scan. His doc said that his stomach may have been in the way during the scan, causing it to look like a blockage. To be sure, he now wants him to do a coronary angiogram.

My question is this: Why would a cardiologist choose an invasive procedure like a coronary angiogram over a CTA? Wouldn't you want to start off with the non-invasive procedure first?