Any HE or low explosive for that matter seeks the easiest escape, why when HE (which TNT is) is used in ordnance it is uniformly contained, for controlled and predictable results. Not a proof for TNT or any other HE.
Because I believe that’s all he had available.
For the greatest effect, TNT needs to be contained, yes, but his goal was not to kill people yet bring destruction.
Lead Azide or DDNP are very hard to obtain. Cheap but hard to obtain with an expired License.