You need proper lighting to prune trees correctly.
1. You need to look out and check for diseased branches and
leaves.
2. You need to do a proper pruning to ensure growth and adequate
air circulation.
3. You need to prune in such a way as not to cause a tree to be
unbalanced or it will fall.
4. You need adequate lighting to ensure all debris are properly
cleared and not cause hazards.
5. It is more hazardous for the workers to work at night without
adequate lighting.
etc,etc.
Plus pruning at night will actually be more hazardous to drivers
as vehicle speeds are usually higher, visual acuity is lower and
you've got more idiots driving at night.