It's really difficult to take pictures!
Eros's shape makes it hard to get a zero degree emission angle, and it's mass is oddly distributed, so it's hard to orbit normally. The NEAR spacecraft has to orbit around the terminator in order to keep its solar panels pointed towards the sun.
We want to view the asteroid with the camera at as close to perpindicular to the surface as possible (that's called zero degrees emission angle), and we also want to see everything in stereo, or from two angles, if that's possible.
One way to achieve this is to use longitude scans instead of lattitude scans.