The problem with quantum computers is that they give every possible answer as a series of probabilities. Selecting the correct answer from the range is not simply a question of picking the one with the greatest probability of being right. The favourite doesn't always win in a horse race even though the pundits all predict that it should. You need a much deeper understanding of all the factors that could influence a horse race to make an accurate prediction rather than just going on past performance of the horse and jockey. Quantum computing can potentially consider all the factors simultaneously and arrive at answers that appear improbable in the absence of knowledge of all the input data.
Douglas Adams came closest to describing the issues in terms a layman might understand in the Hitchhikers Guide to the Galaxy series of books.