I always thought that the reason Sheet Rock screws had so little pull out strength had to do with the type of metal they were made out off. Shows how little I know. Apparently, their weakness has to do with the nature of there thread. From McFeely’s list of Standard Thread Styles…..

Double Lead: Two threads are wrapped around the shank, as illustrated by the blue and red colored threads. One revolution advances the screw 2 pitch lengths advantageous on long screws or in situations requiring rapid assembly. Drywall screws typically use this thread since pull-out strength isn¹t as important as speed. These screws lack the pull-out resistance of “Deep” threaded screws. Typically, #8 and #10 screws 4″ and longer use this thread because of the increased strength afforded by the larger shank diameter.

Needless to say, the whole concept of pitch lengths was knew to me as well. This is also from McFeely’s list of Standard Thread Styles….

Pitch is equal to 1 divided by the number of threads per inch. For a screw with 10 threads per inch, one revolution will advance the screw 1/10″ into the wood.