Anyone know the reason why ?

GFCI's can have nuisance trips. Even a thunderstorm nearby can sometimes
trip a GFCI. This could greatly shorten backup battery time for the smoke
detector, or even cause complete failure if the circuit isn't promptly reset
before the battery dies. The smoke detectors should also be on the same
circuit as often used lighting to make it very obvious if the circuit is open.

For similar reasons, it's not a great idea to put a refrigerator on a GFCI
circuit.

Not exactly the same situation, but a similar principal:

The US Electrical Code has exceptions for overload protection for fire
pumps in residential appartment dwellings. The theory is that you
want power to be supplied to the fire pump no matter what, even if it
burning up do to an overload.