These are cheap, easily modifiable, and readily available containers capable of containing great internal pressures before they finally shatter.

When combined with low-grade explosives, such as gunpowder, you get a vessel that can maintain its integrity until combustion of the propellant is complete, which obviates the need for more sophisticated explosives. I've heard these bombs (not confirmed yet, the press is infuratingly slow about these technical issues) were fueled with gunpowder, the same kind used by firearms enthusiasts to load their own cartridges.

Even making gunpowder is easy, We developed some pretty potent stuff in high school, using charcoal, sulfur and a saltpeter substitute we stole from chem lab. The delivery system was empty CO2 cartidges from air rifles.