I think when states changed from monarchies to democracies many people were not educated and could not read because they weren't allowed to go to school -education for everyone has always been an important democratic issue.
So if you didn't allow people to vote if they can't read they would loose a chance to help themselves. (And in some 3rd world countries there have not been so many changes since 1790).

On the other hand I understand your point. It's not good if any moron has the same chance as a considerate person.

But on the other hand (sorry I think this is already my third hand ): who tells who is the moron?