How to explain the KGB’s amazing success identifying CIA agents in the field?

How to explain the KGB’s amazing success identifying CIA agents in the field?
The problem dated from the mid-’70s, the very time that James Angleton, the paranoid head of agency counterintelligence, was at last ushered out of office, to the relief of conscientious officers hitherto cast under a dark cloud of suspicion, their promotion delayed or, worse still, denied, and in some cases entire careers wrecked.