Oddly, many of the same reasons that disqualify you from owning a firearm also disqualify you from voting. It's that sorry old double standard! We can require IDs to vote because that disenfranchises (a fancy word for denies them their rights) but we don't give a darn if someone is disenfranchised from purchasing a firearm.
You don't give a darn if it was okay to sell guns to the mentally incompetent, convicted felons, and violent offenders?
One vote by an incompetent person will be basically harmless.
But one vote by millions of incompetent people gives us... Well, you know.
Did I say that? I think you've been listening to too many NBC News interviews where they retort with "questions" like this.