That's mainly because eddy currents generated by bobbin probes flow parallel to circumferential cracks.
Bobbin probes are also sensitive to ferromagnetic interference generated by tubesheets and support plates of heat exchangers. The large signals generated by these structures usually mask the small signals from circumferential cracks. Bobbin probes are also sensitive to liftoff, such as the one found in expansion transitions. This is where the patented DefHi® eddy current array probe comes in.