Why did God let people think demons cause epilepsy?

[Razzle]

I've already noted that the cases of casting out of demons occurred within a few years. It's easy to conclude that other causes are far more common.

It's easy for YOU to conclude that, only because you've been raised in your particular culture. Why exactly would anyone born before the scientific revolution have supposed that "demons were only cast out for a few years"? It doesn't say that anywhere in the Bible. People had already been casting out demons in other religions for years before that, so they knew that wasn't true, and there's nothing in there that says "OK, Jesus did it, the apostles did it, but from now on it's going to be rare that people with these symptoms have demons. Starting from now, everyone stop assuming that." 

Huh? Are you saying that scientists delayed research into certain disorders because they believed that those things were caused by demon possession? I find that hard to believe.

Not modern scientists, but their ancient and medieval equivalents. People who would otherwise have experimented with medicine until they found things that seemed to help, as they successfully did with pain relief strategies. This would have been possible before we knew much about the brain. Romans with mental illnesses used to bathe in and drink water that we now know contained lithium salts. If it wasn't for superstition, e.g. the Romans attributed it to the god of that particular spring, then clues like that would have been studied far more frequently and extensively than they were. They could have isolated the lithium salts, optimised the dose, and taken the product all over the known world. Polytheistic cultures, despite their own beliefs, did show more interest in experimenting with herbs, such as the one that Aspirin is based on, than their Christian counterparts, because in Christian cultures people became afraid of being accused of witchcraft.

Your argument seems to merely be that people are sloppy readers. The Bible can be difficult to understand in parts and requires careful consideration. As Peter says:

2 Peter 3
15 and consider that the longsuffering of our Lord is salvation—as also our beloved brother Paul, according to the wisdom given to him, has written to you, 16 as also in all his epistles, speaking in them of these things, in which are some things hard to understand, which untaught and unstable people twist to their own destruction, as they do also the rest of the Scriptures.

Personally I don't blame the text when people miss parts of it and go beyond it in other parts. That's user error.

Do you really think it would be too much effort for God, knowing exactly how people would interpret those words, to add a very clear, explicit, unambiguous and hard-to-miss warning? Do you really think there's nothing he could have said to have minimise the damage from the tendency to attribute so many ills, both medical and environmental, life to demons and witchcraft?