Its all to do with the size of the bomb-bay.
The He 111 started out on the drawing board as a civil airliner, so the bomb bay had to be added on to the original design at a later date, but due to this, the bomb bay was quite small.
In an effort to get around this, they carried the bombs in 8 cells on their ends in order to fit more in.
When the He 111 started to carry larger heavier bombs these had to be fitted on pylons under the wing roots.
ps, most bombs in WW2 were fused by a small rotating fin on either the tail or nose of the bomb after they left the aircraft.
Before then, a crash or any other type of shock wouldn't (normally) set them off.
