This is a basic design question that I've had a hard time finding an answer to. Why can't manufacturers make a super or hyper-cardioid mic that doesn't have that "bubble" at 180 degrees? This would give the obvious benefit of having a mic that would reject feedback no matter where the monitors are. I suspect that there is some type of physical limitation with the design process. Could someone please elaborate on the reason?
Because acoustical physics won't allow this "improvement". There is no simple layman explanation. The answers lie in the physics and math of mic design. If you have a heavy math background, the equations can be found in the book: "Microphone Engineering Handbook", by Michael Gaylord, ISBN 0-7506-1199-5