And they suck. Everyone who's ever worn one knows they suck. If they're too big they completely kill all sensation, if they're too small they cut off circulation, leading to, you guessed it, total loss of sensation. Topped off with the fact that they fail during higher friction activities (you know, exactly the kind that drastically increase HIV transmission risk), they're never going to be an adequate solution, because there will always be people who'll risk it, because what's the point in having sex if you're not going to feel it anyway?