Because goodmornings are an assistance/ancillary exercise, and until linear gains are exhausted on the simple program approach, assistance exercises are not necessary or productive. Just like for the rest of the program. Actually, those numbers are a little liberal, and probebly should be adjusted upwards to a squat of 365. I'll make that correction in the next printing.
If you have a "core", you've been watching too many infomercials or talking to too many PTs.