Because regulation has lost touch with reality.
I shoot antique guns, hence the gunpowder in my garage. Nobody ever had an accident with gunpowder in their garage but the regulations for storing it went insane under NewLab. It used to be you kept it in a cardboard case you could grab if the house caught fire. It comes in a cardboard case. If you couldn't get it out in the event of a fire it would soon go "Whoosh". Now the firemen will probably stand well back waiting for the time delay bomb to explode before tackling the blaze. Why does it have to be padlocked shut in a 16mm thick plywood case, subdivided to hold individual containers no more than two thirds of the overall height, with an intumescent seal and all chained to the building? Did nobody realise that if you contain gunpowder it explodes, uncontained it merely burns. Why not reclassify it as a propellant, calling it an explosive does seem rather historic and makes it hideously expensive due to the shipment and storage regulations. If you want to save a bob or two, once you get an explosive certificate for gunpowder you fax it to the Health and Safety Executive who post you a Recipient Competent Transfer Authority, no questions asked. You could do away with that.