In my understanding, and in very simplified terms... Due to the strict immigration laws in the US, and the frequency that crew (N.B. whether this is cabin crew, cruise ship crew or other) visit the US, they have to have Visas which link them to the company they work for...
B1/B2 : This visa is required by any crew that will be operating into and more importantly out of the US. If you have any length of layover and are not returning back on the vessel (whether it be Aircraft or Ship) the next time is leaves the country, this Visa permits you to operate out of the US.
C1/D : This visa entitles you to enter the country with the arrival of the vessel, and states you will be residing in the accommodation agreed on your visa application. However, the condition of this Visa is that you depart the US on the next sailing/flying of the vessel you arrived on, hence needing the B1/B2...
Aircraft rarely stay on the ground in the US awaiting the arrival of the same crew to return with it. Cruise ships often dock at least overnight, and the crew leave when the ship does, hence ship crew only require a C1/D.
I think I've made that as clear as mud... sorry!
Regards
BoB