Its because apparently pirates don't blow up ships and when a ship blows up the wreckage magically doesn't contain any cargo. In other words the usual people moaned and moaned and moaned some more so now if we are to play a pirate we are to do so in a gentlemanly fashion.
Oddly enough it wasn't like this in the original Elite of 1984.
You may be correct.
From the 1984 Elite manual.
"Pressurized cargo canisters are the Universal means of storing cargo for Interplanetary Space voyaging. Made of HiFlux Chromon-alloy, they hold one Gal Tonne of goods, under variable pressure and temperature conditions. Tales have been told of such barrels being discovered after over 500 years on barren moons, and such ‘Moon salvage’ is a remarkable source of historical artifact material."
In other words, they're tough.
Not like the wimps of today! Who just [sulk].