There's also the issue of how often those items are used in crimes.
As was pointed out, explosives are mostly used for stump removal, dam bursting, prospecting, and other mundane things all the time, while their use in actual crimes is much less so.
Also, The War On Drugs is a big political hot button topic and that influences a lot as well.
Finally, the restriction on the items in question don't usually impact on John and Jane Q. Public that much (How often do you buy cough medicine in bulk?), while a farmer buying a crate of boomsticks having to jump through hoops would make for some really pissed off farmers, who are a tight-knit community and vote.