I understand why they kidnapped Picard, but why did the Borg ignore the rest of the Enterprise-D in the Star Trek TNG episode "BOBW"? Why not assimilate all on the ship? They only assimilate Picard.
There isn't a specific trope for something like this besides maybe "The Show must go on", So I will call it,
The Plot Must go on!
If the Borg just blew up/killed/assimilated everyone on board, there wouldn't be a show.
The reasoning for the Borg to take Picard was to use him as a spokesperson when dealing with humanity and the Federation and getting valuable data from him. The rest of the crew was irrelevant.
They had a specific goal and ignored the rest. Which is typical Borg.