Because it is a
concession allowed for wet runway takeoffs, otherwise viable weights could not be carried.
The "Normal" rules, i.e. Dry Runway operations require that one reserve means of stopping the aircraft be available. The same rules require a screen height of 35 feet, whereas a further wet runway concession is allowed reducing this to 15 feet.
These two wet runway concessions eliminate entirely
ANY margins for Accelerate-Stop performance, and reduce to less than half the screen height required for the GO case.
And yet, there are still jerks out there who would use wet runway data for dry runway operations.
Regards,
Old Smokey