The inherent problem with your idea is the fact that somebody has to program the machine to weight certain responses as being correct or incorrect. While we attempt to use computers to be non-biased, we still run into the situation that standardized tests run into.

No matter how carefully graded, or how carefully constructed, human nature will always bias tests on their own personal experiences. For the SAT/ACTs, this means that the people who will do best are of the same social status of the framers of the tests.

For the electoral computer, even more problems would come up. Do we discount someones vote because they can't use proper articles, which demonstrates low intelligence? Should we judge their intelligence? Is it acceptable to simply vote along party lines (like most of Europe)?

There are just too many "what if"'s and biases to ever put a condition on voting. If we wanted to have Communist elections then we'd just put a gun to everybody's head.