Because there's room for misuse and possible aggravation. A woman who wants to play a cruel prank can seduce a man, but use one of them (according to my sister's friends, they're called "rape condoms" but go figure) and cause serious pain and problems for the male, as they're only surgically removable without totally fucking up your dick.

In addition, during a gangrape, said device would get removed after the first case of penetration, as the device latches on to the person's genitalia. This leaves a group of angrier men than before, which could result in a more violent reaction. Rape isn't entirely about the sexual aspect of it, but about the power assertion. By thwarting the penetration, you just set yourself up for an angry beating from the perpetrator.

And the only way these devices are useful is if a woman can get them without question. See reason one for why that's an issue. It'd be pretty weird for a woman to go up to somebody and say "I think somebody is going to try to rape me in the next 24 hours, so can I please have the equivalent of a bear trap to stick into my vagina?" Exaggeration aside, there's too much hassle and problems that arise from them.

A better solution would be to stop the perpetrators from becoming rapists through national education (sex ed classes yo), as preventative measures work better than reactive measures by a long shot.