My theory was that they calculate risk and can determine how many years on average one will go without a claim and how much that claim will cost. When you have too many or none, then your risk level is outside of the determined norm and you become a bigger liability for a large claim so they drop you. Or, data tells them that you will have a claim in 4 years but you make it 4 clean then they drop you because statistically it is going to happen and at that point the odds are not in their favor.
I would suspect that this has something to do with it. They are only making money as long as you are paying your dues and they aren't shelling out anything (or very, very little). And if a particular industry leans to big payouts, a lot of payments, or some combination thereof, they may not want to deal with any of that.
If you are keeping big ticket items that belong to other people for any length of time, statistically the longer you keep those items, the more likely something can happen on your watch. And after so long, chances are something is going to happen. Do anything for any length of time, something is bound to happen.
Having said that, I don't know of a plain "general liability" insurance coverage that would cover something like this, but I haven't had a call to shop around for it to cover anything on those lines. I would imagine something would have to be tacked on to a "general" policy.
If I recall correctly, this is also a home based business (I think you had an other thread on this topic), that may also add in a different layer to how an insurance company would handle a policy like this. I don't know for sure, just speculating.