on the new class b 2 wire smoke detectors why is the common separate.?

Answer 1

Class B ( 2 wire) smoke detectors, do not have a common. It is just two wires. It is a loop circuit.

If you break the circuit, you have a trouble on the line. If you short the loop circuit it goes into false alarm.

The question you should ask is why is it common for a fire alarm to have false alarms?

A Class A circuit has four wires. Two out to the device and two return to the panel.