The nice thing about a thick piece of wood, is that it's actually a pretty good insulator, it's soft (impacts), and while it is flammable, it actually takes a lot for a wooden board to just catch on fire. It also tends to not hold pressure.
A steel cabinet or box is highly conductive, and will transfer heat into the interior very quickly. If it's an ammo can, that's pretty much a bomb.
A plastic cabinet or box is less conductive, but also is still considerably thinner than wood - so will still transfer the heat into the interior quickly. And will melt and pool (and burn even easier), whereas wood will hold its shape, even if on fire.
Best practice for storage is definitely a wooden cabinet. Ideally with primers stored in a separate wooden cabinet. Also, I read once that the reason the 8lb jug is the standard maximum container size for retail purchase, is because if powder goes bad that size is small enough for the heat dissipation rate the be faster than the heat accumulation rate, and the powder will not hit auto-ignite temperature (in theory). In larger quantities, it's higher risk.