You'll find camping cyclinders can travel under certain criteria but someone just chucking one in their case would be caught out at the xray scan...
the point is, MOST people only take personal items in suitcases, whereas MANY boxes tend to have stuff that is not classed as a personal effect (e.g. clothes household good etc) so the risk of something that should not travel being in a box is higher... therefore more screening. Also not every airport has xray screening (think rural/ oil & gas type) so the box check is to weed out many of the 'common' items which go to those places that would normally go by road.
When I was on the handling side of things, it was usual to ask every passenger if they had anything that could be a DG (camping cylinders and Napisan were common examples I gave) which was an additional check that seems to have gone by the wayside in this age of 'online check in' and kiosks. (Though the kiosks do ask, with a diagram, I doubt Joe Public actually reads it)
Hence the dynamite. It belonged to a guy with a shot-firer's cert travelling to a mine. To him it was a 'tool of trade' and not in the least bit dangerous.
Needless to say he had to find a different airline the next time he travelled. (Must add it was down to the questions I had asked that we knew it was in there at all!
)