As underfire noted the reason for several carriers to opt for default F30: fuel.
It is worth noting the fuel different between F30 and F40, when configuring at 4nm, is only ~10kg!
Taken from the same rulebook, using 2nd detent reverse uses 40-70kg (depending on groundspeed at touchdown, auto brake selection and head/tailwind component).
When airlines use fuel as a reason for using F30, it is often not continuing the same argument preferring F40 with an idle reverse capability in favour of F30 with a reverse requirement for the same auto brake selection made.